Ask runners which session made the biggest difference before their first half marathon and most will say the same thing: the long run. It is where your legs, your heart and your head learn to keep going. It is also the session where most people get impatient – and that's where injuries start.
What the long run is for
A long run is simply your longest run of the week, done at an easy, conversational effort. It does three jobs:
- Endurance: more time on your feet strengthens the aerobic system, muscles and tendons for the distance you are training for.
- Fuel: it teaches your body to use fat efficiently and gives you a chance to practise eating and drinking on the move.
- Confidence: knowing you have covered 16 or 18 km in training changes how a 21 km race feels.
How long should it be?
There's no single right number, but a few practical guidelines work for most recreational runners:
- Think in time, not only kilometres. For a half marathon, long runs of roughly 75 to 120 minutes cover most needs. Slower runners reach the time sooner – which is fine.
- For a half marathon, you don't need to run 21 km in training. Many runners peak at 16–19 km. Race-day adrenaline, a taper and a sensible pace cover the rest.
- Keep it in proportion. If your long run is half of everything you run in a week, the week is probably too thin, not the long run too long.
Build it slowly – the new evidence is clear
The old "10% rule" says you shouldn't raise your weekly distance by more than 10%. It was never well supported, and a large study published in 2025 suggests we were looking at the wrong number.
Researchers followed more than 5,200 runners for 18 months, using their GPS watch data. Weekly mileage changes were not clearly linked to injury. What did matter was a single run that was much longer than usual. When one session was more than 10% longer than the longest run in the previous 30 days, the rate of overuse injury went up – by about 64% for spikes of 10–30%, and more than doubled when the run was twice as long as anything recent.
What this means for your long run:
- Grow it in small steps, roughly 1–2 km at a time, and not every week.
- Look back 30 days, not 7. If your longest run this month was 14 km, the next step is around 15 km – not 18 km because you feel good.
- Coming back after a break? Your "longest recent run" is now shorter. Start from there.
How fast?
Easy. Slower than you think, and slower than your race pace. You should be able to talk in full sentences for most of it. The long run trains endurance; speed belongs to other days.
If you're experienced and well recovered, some runners add the last 10–20 minutes at goal race pace in the final weeks before a race. That's an option, not a requirement – and not something to do every week.
Food and drink on long runs
For runs under an hour, most people need only water, if anything. Beyond that, carbohydrates start to matter. Sports nutrition guidelines from the American College of Sports Medicine and two dietitians' associations recommend about 30–60 g of carbohydrate per hour during exercise lasting roughly 1 to 2.5 hours. That's about one to two standard gels, or a sports drink plus a gel, per hour.
Long runs are the place to test this:
- Start eating after about 30–45 minutes, then little and often.
- Try exactly what you plan to use in the race, so your stomach knows it.
- Drink to thirst, more on hot days. Over-drinking is a risk too.
How often?
For most recreational runners, one long run a week is plenty. Every few weeks, make it a little shorter to let your body catch up. And schedule the day after as an easy run or a rest day.
Common mistakes
- Racing the long run. If you finish wrecked, it was too fast or too long.
- Jumping distance to "catch up" after missed weeks.
- Skipping food until you feel empty. By then it's late.
- Doing the long run the day after a hard session. Give hard days space.
The bottom line
Run it easy, grow it patiently, practise your fuelling, and judge your progress over months, not single Sundays. The long run rewards consistency far more than heroics.
Sources
- Frandsen JSB, Hulme A, Parner ET, et al. How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. Br J Sports Med. 2025;59:1203–1210. doi:10.1136/bjsports-2024-109380
- Nielsen RØ, Parner ET, Nohr EA, et al. Excessive progression in weekly running distance and risk of running-related injuries. J Orthop Sports Phys Ther. 2014;44(10):739–747. doi:10.2519/jospt.2014.5164
- Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance (joint position statement of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine). Med Sci Sports Exerc. 2016;48(3):543–568. Full text