Quick Answer
Allow 16–20 weeks minimum. The three key changes are: longer long runs (up to 20 miles), a fuelling strategy practised in training, and a 3-week taper. A marathon is approximately 3–3.5× harder than a half marathon — not twice. Predict your marathon time by doubling your half marathon time and adding 10%.
Half Marathon vs Marathon Training: What Changes
| Element | Half marathon training | Marathon training | Key change |
|---|---|---|---|
| Peak long run | 10–13 miles (16–21km) | 18–22 miles (29–35km) | Significantly longer |
| Weekly peak mileage | 35–50km | 60–85km | ~50–70% more volume |
| Training duration | 10–16 weeks | 16–24 weeks | Longer build required |
| Taper period | 1–2 weeks | 2–3 weeks | Longer recovery needed |
| Fuelling during runs | Optional (some use gels) | Essential (from mile 5–7) | Must be practised in training |
| Long run frequency | Weekly | Weekly, with recovery runs | Back-to-back runs added |
| Recovery time post-race | 1–2 weeks easy running | 3–4 weeks easy running | Much more cumulative damage |
| Mental demand | High (1.5–2.5 hours) | Very high (3–5+ hours) | Sustaining pace under deep fatigue |
For a full breakdown of what each distance involves, our half marathon distance guide and marathon distance guide cover the complete race profiles, average finish times, and what each leg of the race demands.
How Long Does the Half-to-Marathon Transition Take?
8 weeks — possible but not recommended. With 8 weeks, you can just about build the long run from a half marathon level to 18–20 miles and complete a brief taper. There is no room for missed sessions, minor injury, or a recovery week. The training will feel rushed and race day risk is higher.
12 weeks — the minimum most coaches recommend. 12 weeks allows a proper long run progression, one or two 20-mile runs, and a 3-week taper. Still tight, but achievable for a runner who is consistently putting in 40–50km per week after their half marathon.
16–20 weeks — the recommended range for first-time marathon runners. This window allows gradual mileage build (no more than 10% increase per week), sufficient recovery weeks built into the plan, 2–3 peak long runs of 18–20 miles, a proper 3-week taper, and time to practise fuelling during long training runs. Most structured marathon training plans operate in this range. Our running training plans cover the full progression from half marathon base fitness to marathon race day.
The 10% rule — increasing weekly mileage by no more than 10% per week — is the most important structural principle. Violating it consistently is the primary cause of overuse injuries (stress fractures, IT band syndrome, plantar fasciitis) during marathon build-ups. A strong 10K time is also a useful fitness benchmark before starting a marathon build — our good 10K time guide helps you assess where your aerobic fitness sits relative to marathon readiness.
Is a Marathon Twice as Hard as a Half Marathon?
No — and understanding why is essential for pacing and mental preparation.
The distance doubles, but the physiological challenge is approximately 3 to 3.5 times greater. The central reason is glycogen depletion. The human body can store roughly 2,000 calories of glycogen in the muscles and liver — enough to fuel approximately 20 miles of running at moderate marathon pace. A half marathon uses approximately half of this store, leaving meaningful reserves at the finish. A marathon exhausts these stores by mile 20, forcing the body to rely increasingly on fat metabolism — a slower, less efficient energy process — for the final 6 miles. This is the physiological basis of “the wall,” which simply does not occur in a half marathon.
The cumulative muscular damage of 3–5 hours of running also far exceeds that of 2 hours — the eccentric muscle contractions of thousands of additional ground contacts cause progressive micro-damage that compounds over the final miles in a way that doesn’t happen in a half. And the mental challenge of maintaining goal pace under deep fatigue for 45–90 minutes more than you’ve ever run competitively is qualitatively different from anything in the half marathon experience.
The practical implication: If your half marathon felt “hard,” your first marathon will feel harder — disproportionately so. If your half marathon felt “manageable” at goal pace, your marathon will likely feel very hard from mile 20 onward. Plan accordingly. Most coaches advise first-time marathoners to target a finish rather than a time, and to build race experience before setting aggressive time goals.
How to Predict Your Marathon Time from Your Half Marathon Time
The most commonly used prediction formula is: marathon time = (half marathon time × 2) + 10%.
| Half marathon time | Marathon prediction (×2 +10%) | Required marathon pace |
|---|---|---|
| 1:30 (90 min) | ~3:18 | 4:42/km |
| 1:45 (105 min) | ~3:51 | 5:28/km |
| 2:00 (120 min) | ~4:24 | 6:15/km |
| 2:15 (135 min) | ~4:57 | 7:03/km |
| 2:30 (150 min) | ~5:30 | 7:49/km |
| 2:45 (165 min) | ~6:03 | 8:37/km |
| 3:00 (180 min) | ~6:36 | 9:22/km |
Note: This formula assumes adequate long run preparation (at least one 20-mile run) and a practised fuelling strategy. First-time marathoners without these elements frequently run 10–20% slower than predicted — a 3:51 target becoming a 4:10–4:20 finish due to late-race fade. Our running pace calculator generates detailed per-kilometre targets from your half marathon time and goal marathon finish.
The Long Run: The Most Important Session in Marathon Training
The long run is what separates marathon training from half marathon training more than any other element. Half marathon plans typically peak the long run at 10–13 miles (16–21km). Marathon plans require building the long run progressively to 18–22 miles (29–35km), with the peak run typically occurring 3 weeks before race day.
Why the 20-mile run matters: Running 20 miles in training takes most athletes to or near the glycogen depletion threshold — the physiological point where the body must transition to greater fat reliance. Training this threshold teaches the body to use fuel more efficiently, develop the muscular endurance to sustain form under fatigue, and builds the mental resilience of being on your feet for 3+ hours continuously. Athletes who skip the 20-mile run and race their first marathon typically hit the wall harder and fade more severely than those who have trained through this point multiple times.
Long run pace: Marathon long runs should be run at 60–90 seconds per kilometre slower than goal marathon pace — slow enough to hold a conversation throughout. The purpose of long runs is time-on-feet adaptation, not speed development. Running long runs too fast is one of the most common training errors among first-time marathoners — it increases injury risk and extends recovery time without adding proportional fitness benefit.
Back-to-back long runs: Many marathon training plans include a medium-long run the day after a long run. Running on tired legs from the previous day develops the specific fatigue resistance needed for the marathon’s final 10km. A typical back-to-back might be: Saturday 30km long run → Sunday 15km medium-long run. This structure is more specific marathon preparation than longer single runs with complete rest the following day. Speed sessions on the track — 800m and mile repeats at marathon pace or faster — are a valuable complement to long run training during a marathon build. Our guide to finding a running track covers how to locate a 400m athletics track near you for these sessions.
Fuelling: The Biggest Race-Day Differentiator
Many runners complete half marathons successfully with minimal or no mid-race nutrition — the distance can be covered using pre-race glycogen stores if pacing is appropriate. A marathon cannot. This is one of the most significant practical differences between the two distances, and getting it wrong is the most common cause of late-race collapse in first-time marathoners.
When to start fuelling: Begin taking carbohydrate from approximately mile 5–7 (kilometre 8–11), before you feel hungry or tired. By the time you feel you need energy, glycogen depletion is already significantly advanced. Starting early maintains blood glucose levels consistently rather than attempting to recover from a deficit.
How much to take: Most runners need 30–60g of carbohydrate per hour during marathon racing. This equates to approximately one energy gel (25g carbohydrate) every 30–40 minutes. Some runners tolerate more — up to 90g/hour with appropriate gut training — but this requires practise in training. Never race with a nutrition product you haven’t used in training.
Hydration: Sip water or electrolyte drink at every aid station from the start, regardless of thirst. Hyponatraemia (low blood sodium) is a risk for slower runners who drink excessive plain water — if using gels, ensure they are washed down with water, not electrolyte drink, as the two together can cause GI distress.
Practise in training: Every long run over 90 minutes is an opportunity to practise your race-day fuelling strategy. Use the same gels, sports drink, or food you plan to use on race day. Train your gut to accept carbohydrate while running — many runners who don’t practise this experience nausea or GI distress mid-race when they use gels for the first time.
What to Expect at Mile 20: The Wall
The wall — also called bonking — is the phenomenon of sudden, severe fatigue that occurs when glycogen stores are exhausted, typically around mile 18–22 (kilometres 29–35) in a marathon. Not every runner hits the wall in every marathon, but the risk is highest for first-time marathoners, those who paced too aggressively early, and those with inadequate fuelling.
The experience is distinctive: legs that were running adequately suddenly feel heavy, pace drops dramatically, concentration becomes difficult, and even well-prepared athletes can find themselves walking or stopping involuntarily. The wall is not a minor inconvenience — severe glycogen depletion produces a genuinely debilitating drop in performance that can add 30–60 minutes to a marathon time compared to an even-paced effort.
How to avoid it: The three most effective strategies are pacing conservatively (never more than 5–10 seconds/km faster than predicted pace through the first half), fuelling consistently from mile 5 onward, and training the long run to 20+ miles so the body has adapted to running at extended distances. Our tempo run guide covers how to develop the marathon-specific threshold fitness that makes race pace sustainable deep into the race.
The Marathon Taper
The marathon taper — the period of reduced training volume before race day — is longer than the half marathon taper and more psychologically challenging. While half marathon training typically tapers for 1–2 weeks, marathon training requires a 3-week taper following the peak mileage week.
A typical 3-week taper reduces weekly mileage by approximately 20–30% in week one, 40–50% in week two, and 60–70% in the final week before race day. Intensity is maintained — the taper is a reduction in volume, not intensity. Easy runs stay easy. Tempo sessions continue but shorten. Speed work is preserved but not added.
The taper is frequently accompanied by what coaches call “taper madness” — a psychological restlessness caused by the sudden drop in training load after months of heavy mileage. Legs feel heavy, motivation fluctuates, and many runners develop phantom niggles and doubt. This is normal, well-documented, and not a sign of under-preparation. Trust the process and resist the urge to add sessions to feel more prepared.
Are You Ready to Step Up?
Three practical readiness indicators suggest you’re prepared to begin a marathon training build from a half marathon base:
Consistent weekly mileage: You’re regularly running 40–50km per week in the months following your half marathon, with no more than 1–2 forced rest weeks due to injury. Runners with gaps in consistency after a half marathon should rebuild their base before beginning a marathon build.
No injury issues: The increased volume of marathon training amplifies any existing biomechanical issues or overuse injuries. Address niggles before beginning — IT band tightness, Achilles soreness, and plantar fasciitis all respond poorly to sudden increases in mileage. Our gym exercises for runners guide covers the strength work that prevents the most common running injuries during high-volume training periods.
Your half marathon felt manageable, not maximal: If your half marathon felt like an all-out effort that took days to recover from, your aerobic base may not yet support the demands of marathon training. A runner who finishes a half marathon feeling they could have continued more comfortably is in a better position to step up than one who barely survived the final 5km. Use your half marathon time in our pace calculator to assess whether your aerobic capacity is ready for marathon pace training.
Ready to Step Up to the Marathon?
SportCoaching's marathon training plans are built specifically for runners stepping up from the half — structured long run progressions, integrated strength sessions, and fuelling guidance from an Olympic-experienced coach.
FAQ: Half Marathon to Marathon
How long does it take to train from a half marathon to a marathon?
Minimum 12 weeks, but 16–20 weeks is strongly recommended. You need time to build the long run from ~13 miles to 20 miles, include 2–3 peak runs, and complete a 3-week taper. Don’t rush this step-up — most first-marathon injuries occur from inadequate build time.
Is a marathon twice as hard as a half marathon?
No — approximately 3–3.5 times harder. Glycogen depletion at mile 20 creates “the wall,” which doesn’t occur in a half. Cumulative muscle damage, time on feet, and mental fatigue all increase disproportionately beyond the half marathon distance.
How do I predict my marathon time from my half marathon time?
Double your half marathon time and add 10%. A 1:45 half predicts approximately 3:51 marathon. First-time marathoners often run 10–20% slower due to fuelling errors or insufficient long run preparation. Use the pace calculator for detailed splits.
What is the biggest difference between half marathon and marathon training?
Three things: the 20-mile long run (half marathon peaks at 13 miles), the fuelling strategy (essential from mile 5 in a marathon), and the 3-week taper. Weekly mileage also increases by roughly 50–70% compared to half marathon peak training weeks.
What is the wall in a marathon and how do I avoid it?
The wall is severe fatigue caused by glycogen depletion, typically at mile 18–22. Avoid it by pacing conservatively (not faster than predicted pace), fuelling with carbohydrate every 30–40 minutes from mile 5 onward, and completing at least one 20-mile training run before race day.
Find Your Next Running Race
Ready to put your training to the test? Here are some upcoming running events matched to this article.
