Winter is (eventually) coming, but the long run always pays its debts
Pretty soon we will actually want to go one more, but new evidence shows us why maybe we shouldn’t
It takes a great deal of bravery to build your running base in the Summer
If you're like most of my athletes, you are currently languishing in Summer base building, starting to take a peak at your fall marathon goal mileage, and thinking there is no way I'm running multiple 20+ milers this fall. I’ve got bad news, good news, and medium news. The bad news is that you’re probably not going to feel much better for another month. The good news is that this is expected, it has nothing to do with your fitness, and better days are ahead.
The medium news is that when those better days arrive — and suddenly it’s October and you’re on your long run and you feel invincible and unstoppable — it is important that you are in position and prepared to really take advantage of those days, and not end up on IR instead.
Fortunately, new evidence is giving us the clearest guidance yet on how we do that.
The One Metric to Rule Them All
Last Summer, Frandsen and colleagues published results from the Garmin-RUNSAFE Running Health Study — an 18-month prospective cohort that tracked 5,205 runners across 87 countries, logging more than 588,000 training sessions via GPS.
Amongst other things, the researchers set out to answer a deceptively simple question: which measure of training load increase best predicts injury? They looked at three:
How much your single-session distance increases relative to your longest run in the past 30 days
Your acute-to-chronic workload ratio (ACWR) — weekly volume versus the prior 3-week average
Simple week-to-week mileage change
The results were striking. The ACWR and week-to-week mileage changes showed no meaningful predictive relationship with injury. The only metric that mattered was the single-session spike — specifically, how much longer any given run was compared to your longest effort in the preceding month.
Once a single run exceeded 10% of that 30-day longest run, injury risk rose significantly:
10–30% longer than your longest recent run: 64% higher injury risk
30–100% longer: 52% higher risk
More than 100% longer: 128% higher risk
These findings don't come out of nowhere — the 10% weekly rule has long been on shaky ground and criticism of the ACWR (often built into running app algorithms) has been growing for years. Also, this study is far from perfect: the cohort skews older, male, and experienced; intensity and terrain weren't captured; and injuries were self-reported rather than clinician-confirmed. But what stands out from this study is the sheer scale: 5,200 runners, 588,000 GPS-tracked sessions, and a direct head-to-head comparison of all three metrics in the same dataset — providing some of the strongest evidence to date on what really matters most when building towards your marathon.
It’s the questions we can’t answer that teach us the most
The goal of the next few weeks isn't just to survive summer base building — it's to arrive at your fall cycle positioned to actually capitalize on the fitness you've been quietly building. That means treating the long run with a level of intentionality. Here's what this research tells us:
The long run is your primary risk variable. Not weekly volume (though this should still be managed carefully). Not day-to-day fluctuations in easy mileage. The long run is the lever that matters most, which means it's also the one we need to move most deliberately.
Mileage increases belong on your midweek runs first. When we add volume, we add it across the week before we add it to the long run. The long run should feel like a slow, intentional ratchet — never a jump.
When life gets in the way, don't try to make up for it in one run. A couple of down weeks don't erase your fitness — but going out and running 18 miles to "get back on track" after two weeks of 8-mile long runs is exactly the kind of single-session spike this research flags as high risk. Step back in at a reasonable distance, rebuild the anchor over a week or two, and you'll lose far less time than you would to an injury.
Remember: with great suffering comes great aerobic capacity
Summer heat and humidity make effort-based comparisons unreliable. An 11-mile long run in 85°F is not the same physiological event as an 11-mile long run in October. You really are building the aerobic and structural foundation right now. Fitness is still developing. The speed and ease will show up later, I promise.
The goal between now and the start of your fall cycle is simple: get the long run to a place where the first long run on the schedule feels unremarkable. Not heroic. Unremarkable. Build that platform steadily, and when October finally delivers on its promise, you'll be ready to meet it.
May your long run progression be more consistent than my fantasy references, and may the odds be ever in your favor.
— Coach Matt