Why Do Runners Get Injured So Much?

Increasing your long run by even 10% could raise injury risk by over 50%. So how are you supposed to build mileage safely?

A 2025 study from Aarhus University tracked Garmin data from over 5,000 middle-aged recreational runners for about a year and a half. Injuries were anything that made them change training — not every niggle, but pain that actually changed behavior.

They found no link between how much weekly mileage went up week to week and injury risk. That matches other research: total weekly mileage progression does not track injuries very well.

What did matter was the long run. If a single session was more than 10% longer than the longest run in the previous 30 days, injury risk went up. Weekly mileage could stay flat while the long run jumped — and that still raised risk.

Think of three bands relative to your longest run in the last month:

• Low risk: long run increases under 10%
• Moderate risk: roughly 10% up toward doubling it
• High risk: more than doubling (over 100% longer)

None of these are zero risk. Keeping the long run steady is lower risk, not no risk.

The practical takeaway: pay more attention to how fast the long run progresses than to the classic weekly 10% rule. Look at your longest run in the last 30 days, then check what is coming up. Strength work still matters too — stronger tissue handles those long-run stresses better.

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