Long Runs Explained — How Long, How Slow, How Often
The long run is the most important session in distance running — and the simplest: one run a week, longer than the others, run slowly. It builds the endurance that carries the second half of any race, teaches the body to burn fat efficiently, and hardens the legs for hours of impact. Everything else in the week supports it; nothing replaces it.
This guide explains how long the long run should be, how slow it should be, how often to do it, how to fuel it, and the mistakes that turn a useful long run into a wasted one — or an injury. It is written for runners from 10K to marathon, because the principles are the same at every distance, and only the numbers change.
Use the calculator
Start with your own numbers: try our marathon pace calculator, running calorie calculator to work out your figures before applying anything on this page. The worked examples below use round numbers so you can follow the maths; the calculators use your real weight, height, age and goal.
Related reading: Marathon Training Guide — 16-Week Plan, Pacing and Fuelling, Easy Runs Explained — Why Slow Running Makes You Faster, How to Recover After a Marathon — The First 4 Weeks. You can also browse the full guides section for the rest of the series.
The key points
The long run should be 25–30% of your weekly running time. For a runner doing 4 hours a week, that is 60–75 minutes; for a marathoner doing 7 hours, it is 2–2.5 hours. Beyond 30%, the long run becomes so long relative to the rest of the week that recovery suffers and injury risk rises. The long run is the biggest session, not the only session — the rest of the week still matters.
Long runs are always easy — conversational pace, never raced. The long run is run 45–60 seconds per kilometre slower than marathon pace, or 60–90 seconds slower than 5K pace — slow enough to speak in full sentences throughout. Running the long run fast is the most common error in distance training: it turns an endurance session into a race, leaves the legs too tired for the following week, and adds injury risk without adding endurance. The long run is slow by design.
Build the long run by about 10% a week. From 60 minutes, add about 6 minutes the next week, not 30. The body adapts to gradual load; it breaks under sudden load — and the long run is where sudden load does the most damage, because it is the longest exposure to impact in the week. Every third or fourth week, cut the long run back by 25–30% (a cutback week) to let the body absorb the training.
The long run teaches fuelling. For runs over 90 minutes, take carbohydrate during the run — 30–60 g per hour from gels, chews or drinks — and water throughout. The gut needs training just as the legs do: a gel that works on a long run is safe on race day, and a new product on race day is a gamble. Practise the exact products, the exact timing, and the exact water strategy on the long runs, so race day holds no surprises.
The long run is a mental rehearsal. Hours on your feet teach patience, pace judgement, and the skill of running when tired — which is exactly what the second half of a race demands. Runners who have done the long runs arrive at the start line knowing they can cover the distance; runners who have not arrive hoping. The confidence is not a bonus; it is one of the main products of the long run.
The peak long run for a marathon is 30–35 km, three weeks before the race. Longer than that adds fatigue without adding fitness — the body cannot recover from a 40 km training run in time to benefit from it, and the injury risk rises sharply. The marathon itself is the longest run; the training peak is deliberately shorter. For a half marathon, the peak long run is 16–19 km; for a 10K, it is 12–15 km. The peak is a proportion of the race, not the race itself.
At a glance
| Race | Peak long run | Long-run pace | Long-run duration |
|---|---|---|---|
| 10K | 12–15 km | Easy, conversational | 75–90 min |
| Half marathon | 16–19 km | Easy, conversational | 1.5–2 hours |
| Marathon | 30–35 km | Easy, conversational | 2.5–3.5 hours |
| 5K (maintenance) | 8–10 km | Easy, conversational | 45–60 min |
The peak long run comes 2–3 weeks before the race, followed by a taper. Never run the peak long run in race week.
The formula behind it
The long-run pace is calculated from race pace: long-run pace = marathon pace + 45–60 seconds per kilometre, or 5K pace + 90–120 seconds per kilometre. A runner with a 4-hour marathon goal (5:41 per km) should run long runs at about 6:26–6:41 per kilometre; a runner with a 25-minute 5K (5:00 per km) should run long runs at about 6:30–7:00 per kilometre. The Riegel formula (Riegel, 1981) predicts race times from a known performance — time₂ = time₁ × (distance₂ ÷ distance₁)^1.06 — and it is a useful check that the long-run pace and the race goal are consistent.
The fuelling maths is the reason long runs need food: the body stores about 2,000 kcal of glycogen, and running burns roughly 1 kcal per kilogram per kilometre — so a 75 kg runner burns about 75 kcal per km, and 2,000 kcal covers about 27 km. A long run beyond 90 minutes (roughly 15 km) starts to deplete glycogen, which is why carbohydrate during the run (30–60 g per hour) matters. The calorie maths is the Compendium (Ainsworth et al., 2011): running at 6 mph (9.8 MET) burns 9.8 × weight (kg) per hour, so a 2.5-hour long run at 75 kg burns about 1,838 kcal — a large part of the day's energy, and the reason the post-run meal matters as much as the run itself.
Your plan, step by step
- Schedule one long run a week, on the same day, at 25–30% of your weekly running time.
- Run it easy — conversational pace, 45–60 seconds per km slower than marathon pace — and never race it.
- Build it by about 10% a week, with a cutback week (25–30% shorter) every third or fourth week.
- For runs over 90 minutes, take 30–60 g of carbohydrate per hour and water throughout, using race-day products.
- Peak 2–3 weeks before the race (30–35 km for a marathon), then taper — the long run is done, and the fitness is banked.
How to use this guide
Use the long run as the anchor of the week: plan the other sessions around it, keep the day before it easy, and keep the day after it easy or rest. The long run is the session that the week is built for, and the sessions around it exist to support it — not to compete with it. A week with a good long run and mediocre other sessions beats a week with no long run and excellent other sessions.
Treat the long run as a rehearsal, not just a workout: same shoes, same kit, same food, same drink strategy as race day. The rehearsal is where problems are found — the gel that upsets your stomach, the sock that blisters, the route that has no water — and finding them in training is free, while finding them on race day costs minutes. The long run is the dress rehearsal, and the race is the performance.
What this guide does NOT tell you
The long-run guidelines are population averages: the right length, pace and build rate depend on your fitness, age, training history and the race, and the 10% rule is a guideline rather than a law. The glycogen figures are estimates — individual stores vary with diet, training and body size — and the fuelling recommendations are general, not personal prescriptions.
This guide is general information, not medical advice. Long runs are the highest-injury-risk sessions in distance training, because they are the longest exposure to impact, and they should be built gradually from an established base. Anyone with a heart condition, joint problem or medical condition affecting exercise should check with a clinician before starting long-run training, and chest pain, severe breathlessness or sharp joint pain during a long run are signals to stop and get checked.
Frequently asked questions
How long should my long run be?
About 25–30% of your weekly running time, built gradually: 60–90 minutes for a 10K runner, 1.5–2 hours for a half-marathoner, and 2.5–3.5 hours (30–35 km) at the peak for a marathoner. The peak comes 2–3 weeks before the race, followed by a taper. Longer than 30% of the week, or longer than 3.5 hours, adds fatigue and injury risk without adding fitness — the body cannot recover from it in time to benefit.
How slow should a long run be?
Slow enough to speak in full sentences throughout — about 45–60 seconds per kilometre slower than marathon pace, or 60–90 seconds slower than 5K pace. A 4-hour marathoner (5:41 per km) should run long runs at about 6:26–6:41 per km. Running the long run fast is the most common error in distance training: it turns an endurance session into a race and leaves the legs too tired for the following week.
Should I eat during a long run?
Yes, for runs over 90 minutes: 30–60 g of carbohydrate per hour from gels, chews or sports drinks, plus water throughout. The body stores about 2,000 kcal of glycogen — enough for roughly 27 km — so a long run beyond that depletes it, and the carbohydrate you take during the run is what prevents the wall. Practise the exact products on the long runs, because the gut needs training too.
How often should I do a long run?
Once a week, on the same day, with a cutback week (25–30% shorter) every third or fourth week to let the body absorb the training. More than one long run a week is possible for very experienced marathoners, but for most runners a single weekly long run is enough — the rest of the week is easy runs, one tempo and one interval session. The long run is the anchor, not the whole week.
What if I can't finish my planned long run?
Stop, walk home, and treat it as information, not failure: the plan was too ambitious, the pace was too fast, the fuelling was wrong, or the week was too tiring. Cut the next long run back by 20–25% and rebuild more slowly. A failed long run is a normal part of marathon training — almost every marathoner has one — and the correct response is a smaller next run, not a bigger one to "make up for it".
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.