Why Is the Last 10K of a Marathon So Hard? (And How to Get Through It)
Running a marathon is tough from start to finish. But ask almost any runner, and they’ll tell you the same thing: the last 10K is where it really starts to hurt. Legs feel heavier, your pace starts to slip, and even holding form can feel like a challenge. So why is the last 10K of a marathon so hard?
It’s not just in your head. There are a few very real physical and mental reasons why those final kilometres feel so much tougher — and understanding them can make a big difference on race day. It’s something I’ve experienced myself in races, especially in the later stages of a marathon when even maintaining pace starts to feel like a challenge.
Why the Last 10K of a Marathon Feels So Much Harder
By the time you reach the final 10K of a marathon, your body has already been working for hours. At that point, multiple systems start to break down at once — and that’s what makes this part of the race feel so different from everything that came before.
Here are the main reasons it happens:
1. Your Glycogen Stores Are Running Low
By the last 10K, you’ve likely burned through most of your stored carbohydrates.
Even if you’ve been fuelling consistently during the race, it’s hard to fully keep up with the energy demand of running 42.2 km. As those energy stores drop, your body is forced to rely more on fat — which is a slower energy source.
That’s why your pace can suddenly feel harder to maintain, even if you haven’t changed anything. This is also what runners often refer to as “hitting the wall” — when your energy levels drop and everything suddenly feels much harder.
2. Muscle Fatigue Starts to Catch Up
At this point, it’s not just your lungs working — your muscles have been absorbing impact for hours.
Each stride creates small amounts of muscle damage, and by the final 10K, that fatigue has built up enough that it starts to affect how efficiently you run.
This is when your legs start to feel heavy, your stride shortens, and everything takes more effort than it did earlier in the race.
3. Your Form Begins to Break Down
As fatigue builds, it becomes harder to maintain good running form. You might notice your posture slumping, less arm drive, and shorter, less efficient strides.
Even small changes in form can make running feel significantly harder — especially over longer distances.
4. Your Brain Is Trying to Slow You Down
This is the part a lot of runners don’t expect.
As your body gets more fatigued, your brain starts to act as a kind of “protective system,” increasing your perception of effort (how hard the effort feels) to stop you from pushing too far. This is closely tied to how your body regulates exercise intensity and effort during prolonged activity.
That’s why the same pace can suddenly feel much harder — even if physically, you’re still capable of maintaining it.
5. Pacing Mistakes Earlier in the Race Show Up
If you went out slightly too fast (which is very common in marathons), the last 10K is where it catches up to you.
Even a small pacing error early on can lead to:
- faster glycogen depletion
- greater muscle fatigue
- a much tougher final stretch
This is often why experienced runners say the marathon really only starts after 30–32 kilometres (18–20 miles).
How to Make the Last 10K Feel More Manageable
The last 10K of a marathon is never going to be easy — but there are a few things you can do to make it more manageable.
- Pace conservatively early: Going out slightly slower than goal pace can pay off massively later.
- Stay on top of fuelling: Consistent carbs and electrolytes help delay fatigue.
- Break it down mentally: Instead of thinking “10K to go,” focus on smaller chunks — aid station to aid station works well.
- Expect it to be hard: Knowing this is coming can actually help you handle it better when it hits.
Finally, one of the most important things you can do leading up to a marathon is to practice your long runs. Your long runs are where you build the endurance and resilience needed for the final 10K. Finishing your long runs on tired legs helps prepare your body (and mind) for what the last part of the marathon will actually feel like.
The Bottom Line
If the last 10K of a marathon feels disproportionately hard, it’s completely normal.
By that point in the race, your body is dealing with low energy stores, accumulated muscle fatigue, and increasing mental effort — all at the same time.
The good news is that with smart pacing, proper fuelling, and the right mindset, you can handle it far better — and even cross the finish line feeling strong.
