How Much Running Is Too Much? What a New Study Means For Running Injury Risk
Runners in Boston know that training smart is the key to staying healthy through marathon season, local road races, and those daily Charles River miles. For years, you’ve probably heard about the “10% rule” — don’t increase your total weekly mileage by more than 10%.
But a new 2025 study by Frandsen et al. (British Journal of Sports Medicine) challenges this idea and shifts the conversation about running injuries and mileage increases.
The Study: What Puts Runners at Risk?
Over 5,200 recreational runners were tracked for 18 months. Researchers wanted to know: When does increasing your mileage actually increase your chance of injury?
✅ The big takeaway: single-run mileage spikes — not total weekly mileage — were the strongest predictor of overuse injuries.
Why Long-Run Spikes Are the Real Culprit
The study found that when a single run was 10% or more longer than a runner’s longest run in the past 30 days, the risk of running injury jumped significantly. For example:
10–30% increase = ~64% higher injury risk
30–100% increase = ~52% higher injury risk
100% increase (doubling your longest run) = ~128% higher risk
That means if your longest run in the past month was 8 miles, jumping to 10, or more without a build-up could tip the scales toward an injury.
By contrast, week-to-week mileage increases showed little correlation with injury risk. This suggests the “10% rule” for weekly volume may be less important than paying attention to your long run progression.
Who Is Most at Risk?
You might be thinking: “Keeping increases under 10% feels painfully slow, especially when training for a marathon.” And you’re not wrong. Many popular training plans, even beginner-friendly ones like Hal Higdon’s marathon plan, include bigger jumps — for example, moving from 6 miles → 7 miles → 5 miles → 9 miles in the first four weeks. That’s a 17% increase one week and a 29% increase the next. These don’t always feel drastic, but they still increase risk.
This is especially tricky for busy Boston runners, like moms and dads fitting in early-morning or lunchtime runs. One week it’s your usual 4-mile loop, then suddenly you find a quiet Saturday and double your distance with an “easy” 7-miler. That’s a 75% jump compared to your baseline — and a red flag for injury according to this study.
For postpartum runners or those returning from injury, the challenge is even greater. If your longest run has been 2 miles, jumping to 4 may feel fine — but it’s a 100% increase, linked with a 128% higher injury risk in this study. These phases often feel slow and frustrating, but they’re when caution matters most.
👉 The good news? You can safely build total weekly mileage with shorter, more frequent runs, as long as your long run increases stay gradual.
Yes, this approach may feel like a crawl toward marathon-level mileage. If runners followed the 10% rule to the letter, an 18-week marathon plan could stretch into 26 weeks. But this study doesn’t mean progress has to stall — it means we should train smarter and harness this knowledge to make educated training decisions, build stronger bases, and make thoughtful long-run progressions.
What This Means for Boston Runners
Whether you’re preparing for the Boston Marathon, training with a local club in Somerville, South Boston, or Cambridge, or rebuilding fitness after time off, the message is clear:
Build gradually → Spread mileage increases across multiple runs, not just your Sunday long run.
Respect your 30-day history → Check what your longest run has been in the past month before adding distance.
Don’t chase hero runs → One big jump can derail an entire training cycle. Consistency wins.
Work with a physical therapist → At The Run Rx in Boston, our physical therapists and running specialists help athletes prevent overuse injuries, recover faster, and train smarter.
Injury Prevention in Boston: Physical Therapy for Runners
At The Run Rx, we specialize in sports physical therapy — from injury rehab to marathon training support. Our clinic is built by runners, for runners and deeply connected to the Boston running community.
👉 Want help increasing your mileage safely and reducing your injury risk?
Book a free discovery call with our Boston-based physical therapy team today.
Somerville, MA
