Marathoners Beware: One Long Run Mistake Could Double Your Injury Risk

Before you stack miles onto your long run, here’s what the science—and a physical therapist—want you to know.

Let me give no-nonsense answers to your burning questions:

“Can a single long run cause injury?” 

“How much further can I safely run?” 

“Why did I get injured after just one long run?” 

“Why doesn’t the 10% rule ‘work’ anymore?”

But we need to address the elephant in the room.

Going hard-core nerd, there’s been a study floating around for a hot minute now: 

“How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study.”

If you haven’t heard of it, that’s ok.

But if you’re wondering why these questions are getting the spotlight more than usual, this study is why.

Have these questions always been around?

Yes. But this recent study is stirring the pot, resulting in conflicting, or at least new answers, from running coaches and health and movement professionals. 

So if you’re interested, I’m going to give you my two pennies.

And some well-rounded answers with a secret sauce: nuance. 

Quick recap of this study so you’re not lost: 

This study has been recently picked up by running media and community and considered a spicy topic as it sets itself up to be a paradigm-shifting study from Aarhus University, published earlier this summer (2025). The major insight it claims to have: injuries often result from one single run that dramatically exceeds a runner’s recent longest distance, rather than gradual overuse buildup.

Already, I agree with the first part of that statement.

Yes, setting out on a long run that your body is not prepared for could potentially result in injury. It’s a coin flip.

But to say that THAT’S the KEY reason for running injuries rather than a gradual accumulation of tissue damage which we know occurs from work outs?

That’s blatantly ignoring a vast body of literature.

But here’s the kicker. There are “scary numbers” that might be the reason everyone is losing their minds:

The study found:

• Running 10–30% beyond your longest run in the past 30 days increases injury risk by 64%

• A 30–100% spike raises risk to 52%

• Going over 100% farther (doubling your longest recent run) raises risk by 128%

  
  

This study deliberately pitches this in contrast to the older “10% rule” for weekly mileage increases and suggests that real-time, run-by-run monitoring may be more vital than cumulative load tracking.

My two pennies coming back in:

I’m not mad about questioning the 10% rule.

And I would agree, keeping a running log and writing down how each run went is a great way to collect data!

But we shouldn't, under any circumstances, ignore looking at total mileage over 30 days since we can use that data to assess how well your body adapted and make data-informed decisions for the next month. 

So with that little recap and mini ranty-rant complete, let’s answer those questions above.

I’ll sprinkle in some more questions and thoughts I had along the way, but you should know I’ve edited things down quite a bit. Otherwise, this post would be more of a novel rather than a blog. 

Can a single long run cause injury?”

Yes. and No. Remember our secret ingredient: nuance. 

What this study wants to prove is:

“runners may be more vulnerable when increasing distance too rapidly within a single session, indicating that the ‘too much period’ for running injuries may be shorter than the 1 week window commonly used in research and wearable devices.”

They are actively searching to prove the “single session paradigm -whether a spike in km run” can result in an increased risk in injury

What I need you to remember is that standard “running lore” dictates that “a 30% increase….is the weekly averaged running distance [and] exceeding this amount has been associated with increased risk of injury.”

Meaning, a good running coach writes your long as 20-30% of your total weekly mileage.

This means, we already know it’s a bad idea to just double your long run because you feel like it.

Do some runners do it?

Well, they had enough data to show that some of us have tried that.

And it didn’t go so well. 

But as for can a single long run cause an injury? It depends. Are you deciding to increase your long run within the normal range of 20-30% or are you trying to increase it by 100%? 

What I want you to take away from this question-answer set is this:

a good running plan is your friend. If you’re winging it, not tracking your mileage or running by time, then yes, it’s very easy to make a silly, avoidable mistake that could lead to a running injury. 

But it says running even beyond 10% of your long run increases injury risk by 64%???? 

Here’s the catch:

This study has no control. 

It has a TON of unaccounted for variables.

• we don’t even know if these runners were following a well-written plan or just winging it based on how they felt that day. 

• We don’t know how experienced these runners were, if they were beginners, first time marathoners, seasoned ultra runners…

• We have no idea what those runners were doing the other 6 days of the week.

Yes, we have GPS data.

But the kicker is: NOWHERE In the study have they accounted for intensity beyond “the long run”.

Meaning, we have no idea if these runners did hills.

Speed work.

Any easy runs at all.

Only distance. 

So without diving too much into the weeds because we still have other questions to answer, can you start to see how that number looks terrifying BUT… we dont have the full picture by a long shot. So we take it with a grain of salt and use critical thinking. 

. 

Why did I get injured after just one long run?

Remember how we just discussed intensity above, in the form of speed work, hills, long runs…your basic running workouts?

Just because an injury finally “pops up” after one long run doesn’t always mean that particular run was the mechanism of injury, a phrase we use it the PT world to indicate “ah, yes! This is the cause of the injury!”

Look, we don’t always have that in running. In fact, I’ll give this study credit for this statement: 

“...the casual assumptions underpinning the study should be carefully considered…the cause of running-related injury is multifactorial involving various exposures.”

Meaning, a running injury is a story.

And we know how it ends, but we don’t always know or find out how it started.

A lot of times, it’s a well-informed, educated guess.

That’s why when I work with a runner, I ask about their previous 4-6 weeks of training because we know from previous research, that’s a pretty good place to start looking due to tissue tolerance, or how strong various soft tissues are in your body. 

I know this study is trying to show that ONE sole run can be the straw that broke the camels back, and maybe it could be, maybe one specific run could be that mechanism of injury!

But we need to be careful not ignore the past 4-6 weeks of training and how your tissues and body were responding to that, if you were or weren’t strength training using progressive overload, whether you were adequately recovering with quality sleep, fuel, managing life-stress loads etc.

Can you start to see the puzzle pieces falling into place? 

Running injuries are like an iceberg.

And while we like to blame the tip we can see, there’s so much more beneath the surface that we should not ignore. 

Why doesn’t the 10% rule ‘work’ anymore?

One thing I will applaud this study for is challenging the 10% rule.

The 10% rule is a very simplified version (to a fault) of a more encompassing ratio called the Acute Chronic Workload Ratio (ACWR).

The ACWR looks to compare your recent acute workload (last week) to your average chronic workload (over the past 4 weeks) and based on this, increase your mileage in a way that better mitigates injury risk.

This is where the 10% rule comes from: the over simplification of “just increase low and slow by 10% and you’ll be fine”. 

And that may actually work phenomenally for you….IF….you are ONLY increasing distance. No speed. No to minimal hills. No thresholds. Just distance. 

Because what the 10% rule fails to capture is INTENSITY

(you got it: speed, threshold, hills etc).  

So if you’re following the 10% rule while doing other running workouts, yes, you could very well find yourself injured “after just one long run” because of an over all lack of recovery, a general state of over use, compounded by poor fueling…or even lack of fueling entirely on your long run!

So is the 10% a total bust?

No. But it’s also not the full picture. And it flows directly into…

How much further can I safely run?

This is where you need to take into account your ENTIRE running plan.

I need you to set aside the scary “increase injury risk by 64%” because this is a number without full context. 

Your ability to run well, fast, far…are all influenced by a million different factors.

How well did you sleep that week? Have you eaten enough the past few days? Did you bring enough fuel? 

And that’s not even taking into account your years of experience running, if you’re coming back from an injury, and if you’re strength training or not!

Again, this is where we lean on known long run ranges that are tried and tested.

But I would extend that 20-30% to even 10-40% of your weekly mileage. 

Because different runners have different bodies.

And different running goals.

And while at the beginning of your running journey you may need to lean more into that 10% range, over a couple years, you’ll find yourself in that 30% range (probably with the help of consistent, running-specific strength training). 

WRAPPING UP

I want you to take a look at the Limitations section of this paper. 

Because this clears up a LOT. 

“The following limitations are noted: (1) validity of the selfreported injury outcomes; (2) use of running distance as an exposure and the use of ‘binned’ exposure states; (3) the analytical approach including generalisability; and (4) causal assumptions. Injury assessment, origin (overuse vs traumatic) and location were self-reported, which carries information bias…”

And this part…

“...it was not possible to provide specific advice for different subgroups of runners. Finally, the causal assumptions underpinning the study should be carefully considered. While confounders such as sex, BMI and prior injuries were included, the cause of running-related injury is multifactorial involving various exposures. The observational nature of the study design limits the robustness of the causal claims regarding injury prevention. To evidence-base the preventive effect of recommending runners to stay below a 10%-spike threshold to reduce injury risk, randomised trials or novel approaches such as target trial emulation should be pursued in future work.”

The authors are admitting that they essentially had a crap ton of data, but the results they pulled from it require better, more objective, and evidence-based research to back them up and actually prove THEM TRUE. 

Because right now, this is a very near-qualitative study rather than a true quantitative study with a control group and controlled variables. 

So please, don’t panic.

Realize data can be manipulated to tell the story the author wants told.

Luckily, in this case, these authors at least stated at the end that these numbers don’t tell the entire story. 

What does all of this mean? 

1. Doubling your long run is probably a bad idea

2. You potentially risk injury every time you run longer, but this study completely forgot about the super-powers bestowed on runners who consistently and regularly strength train with running specific exercises. 

3. You still need to increase mileage slowly, but we still don’t know exactly what this magic number is, probably because it’s a little different for every runner.

4. If you want to play it safe because you feel like you’re always injured no matter “how slowly you increase your mileage”, you can try 2 things backed by current evidence: 1) lift heavy like a runner 2) only increase >10% using the ACWR. 

Until next time running fit fam...

Dare to Train Differently,

Marie Whitt, PT, DPT // @dr.whitt.fit

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REFERENCE:

Schuster Brandt Frandsen J, Hulme A, Parner ET, et al. Br J Sports Med 2025;59:1203–1210.