New research · 4 min read · 2026-10-11
How much longer can your long run be?
A Danish-led study of 5,205 runners found more overuse injuries when a run was over 10 percent longer than the longest run in the previous month.
You feel good, the weather is perfect, and you decide to add a few extra kilometres to your long run. Most of us have done it. A large Danish-led study from 2025 suggests it is worth thinking twice.
What the researchers did
Researchers from Aarhus University followed 5,205 runners for 18 months using data from their Garmin watches. That added up to 588,071 runs. Along the way, 1,820 of the runners reported a running injury.
For each run they asked a simple question: how long was this run compared with the longest run in the previous 30 days?
What they found
Runs that were more than 10 percent longer than the longest run of the past month were linked to a higher rate of overuse injuries. Compared with runs that stayed within 10 percent, the rate was about 1.6 times higher when the run was 10 to 30 percent longer, about 1.5 times higher at 30 to 100 percent longer, and about 2.3 times higher when the run was more than twice as long.
The more familiar rules based on weekly mileage did not show the same pattern. A sudden long session seemed to matter more than how much you ran in total that week.
What it means for you
If your longest run in the past month was 15 km, a 16.5 km long run is within 10 percent. A 20 km run is a third longer. That doesn't mean you will get injured. It means the jump is bigger than it might feel on a good day.
Build your long run up step by step. If you have had a break, start from what you have actually run in the last month, not from what you used to run. And if a run grows on the day, keep the next one easy and short.
What the study can't tell you
This was an observational study. It shows a link between big jumps and injuries, not that the jumps caused them. The 10 percent figure is a way of grouping the data. It is not a line where running suddenly becomes safe or dangerous, and your own tolerance depends on your history, your body and how the rest of your training looks.
How we use it
PX7 Training compares a planned run with your longest run in the past 30 days. If the jump is big, the plan flags it, so you can decide whether to run it as planned or keep it shorter.
Source
- Frandsen et al. (2025), How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. British Journal of Sports Medicine.
Written by Jørgen Larsen for PX7 Training.