How far on Sunday: the 10% rule of your own month

Sunday comes round, the morning is free, it is pleasantly cool at eight and your legs are asking for a party. That is where a lot of autumn training blocks break before they even begin: not from running a lot across the month, but from one single day when you got carried away. A study of more than five thousand recreational runners has put a number on that moment, and the number is rather smaller than you would like.

The study: 5,205 runners and half a million sessions

The work was published in September 2025 in the British Journal of Sports Medicine, by Jesper Schuster Brandt Frandsen, Rasmus Oestergaard Nielsen and colleagues. They followed 5,205 recreational runners (mean age 45.8, 22% women) through their GPS watches for up to 18 months: 588,071 recorded sessions and 1,820 people injured through overuse, 35% of the sample.

What makes it interesting is where they looked. Instead of focusing on weekly volume, which is what we all log in the spreadsheet, they compared each individual session, one by one, with that person's longest run over the previous 30 days. Every outing was sorted into four buckets: holding steady or increasing by up to 10% (the reference group), a small spike of more than 10% up to 30%, a moderate spike of 30% to 100%, and a large spike above 100%.

The numbers, unadorned

Compared with sessions that stayed within 10%, injury rates rose like this: 1.64 times higher with a small spike (95% confidence interval: 1.31 to 2.05), 1.52 times with a moderate spike (1.16 to 2.00) and 2.28 times with a large spike (1.50 to 3.48). In other words, doubling your longest run of the month more than doubles your risk of ending up injured.

You will have spotted the oddity: the small spike comes out slightly riskier than the moderate one. It is best not to read too much into that, because the confidence intervals overlap comfortably. What is solid, and what the authors sign off on, is that the jump in risk appears as soon as you go past that 10%. There is no gentle warm-up zone between 10 and 30% before the trouble starts.

The sum you can do right now

The good news is that this takes thirty seconds and a single figure. Open your watch app, filter the last 30 days and find your longest run. That is your reference. Multiply it by 1.1 and you have tomorrow's ceiling.

Examples make it clearer. If your longest run last month was 10 kilometres, this Sunday's ceiling is 11. If it was 14, it is 15.4. If it was 21, it is 23.1. And if you are coming off a quiet month whose longest outing was 8 kilometres, that lovely plan to knock out 16 in one go because "I feel great" is precisely the study's large spike: 2.28 times the risk.

Why this stings more than it looks

The catch is that the ceiling moves with you. Every time you complete a new long run, that becomes your reference for the following 30 days, so the margin grows by itself. With patience, 10% a week takes you from 10 to 16 kilometres in a little over a month. Without patience, it takes you to the physio's table in a single morning.

There is a second, very August-shaped problem: if you eased off on holiday, your 30-day reference eased off with you. Going back in September to the distance you were doing in June is not "picking up where you left off", it is a full-blown spike. Add the heat, which sets a ceiling of its own and which we covered in our guide to running in 35 degrees, and you have the perfect recipe for starting autumn on foot.

What to do with the leftover enthusiasm

Because the enthusiasm is still there, and simply saying "run less" is a bit cruel. If you finish feeling short-changed, put the surplus into something that does not inflate your mileage: a few more minutes of easy running on another day, running drills, or the strength work most of us keep postponing. With twenty minutes, two days and three exercises you cover what matters, and none of it counts as kilometres.

In the end the study's message is almost reassuring: you do not need to run little, you need to avoid jumps. Your progress does not hinge on the morning you got carried away, but on the thirty that come afterwards. And those are earned by staying in one piece.