Forget the 10% rule. Watch the long-run jump

Every running club has the rule. Never add more than 10% a week. It sounds careful. The evidence for it is thin, and it measures the wrong thing.

In 2025 a study followed 5,205 runners for 18 months with their watch data and their injuries. Weekly mileage ratios did not predict who got hurt. A single run did. Runners whose long run jumped more than 10% beyond their longest run of the previous 30 days were at meaningfully higher risk, and the largest jumps carried the highest risk.

The guardrail

Take the longest run you completed in the last 30 days. Multiply by 1.1. That is the ceiling for the next long run. Not a target, a ceiling: plenty of long runs should sit under it, and a cutback week sits well under it.

Why a 30-day window and not last week

Because life happens. A missed fortnight with a cold does not reset you to zero, and the window says so: your longest run from three weeks ago still counts. Equally, one heroic 35 km three weeks back does not licence 38 km this weekend if everything since was short. The window rewards the run you actually completed, not the one you meant to.

What the rule does not cover

• Time on feet. A 25 km run on the South Downs and a 25 km run in the Lakes are not the same load. I prescribe mountain long runs by duration and climb as well as distance.
• Speed. Ten percent more distance at a harder pace is more than ten percent more load. Long runs stay easy for this reason among others.
• Descent. Eccentric load from downhill running has its own recovery cost. The descents note covers it.
• Back-to-backs. Two long runs on consecutive days for a stage race are a separate progression with their own ceiling.

What I watch

• The longest run in the trailing 30 days, updated every week, and how close the next prescription sits to it.
• Whether the long run got done as prescribed, or grew on the day because the athlete felt good.
• Heart-rate drift in the last third of the long run, which tells me whether the distance was absorbed or endured.
• Soreness and sleep in the two days after, reported in the check-in.

In my athletes' plans the guardrail is written into every long-run prescription, and a proposal that breaks it does not reach them. Missed weeks lower the ceiling automatically. Nobody is asked to make up lost distance.

How the long run fits the rest of the week is in ultra running after 40. The Marathon des Sables and Lakeland 100 guides show the progression applied to two very different races.

Evidence level: the long-run jump finding is from the Garmin-RUNSAFE Running Health Study (Frandsen, Nielsen and colleagues, British Journal of Sports Medicine, 2025): 5,205 runners, 18 months, 588,071 sessions, injury self-reported. Strong for an observational study and still one study. The 110% ceiling and the 30-day window are how I apply it; the exact percentage is a coaching convention.