Hitting the Wall in a marathon?

Question

Why do you "hit the wall" in a marathon around mile 20? I know, that's when you supposedly run out of glycogen. But the numbers I've seen don't make sense:

Your body stores about 2000 calories worth of glycogen, and it burns about 100 calories per mile. So 2000 / 100 = 20 miles.

But I thought running at a moderate pace burns a combination of glycogen AND fat. So why do you run out of glycogen at 20 miles? Wouldn't it take longer?

Answer 1

A couple of factors here:
1-2000 cal is the max of glycogen you can have. When the race starts, you're probably down to 1500-1600 as you are burning glycogen all the time.
2-Pace isn't as important as your weight and the difficulty of the course so if you aren't about 150 lb for a male and 110 lb for female, then you use more. Plus every marathon course I've run no matter how "flat" it's supposed to be, is always tougher than advertised.
3-You preferentially use glycogen until it's just about gone. Unless you've really, really trained, using you fat stores is inefficient and you also start to pick off some lean muscle mass when the glycogen's gone. Your blood sugar drops and your leg muscles hurt. Additionally, mentally you are fatigued so the effect adds up.
4-Goo and other carbs during the race can help a little, but a couple of packets will only you gain you a mile or 2.

Answer 2

I ran a marathon when I was 17. I ran mostly 5k and some long ones as 1/2 marathons, but that marathon was my first. The longest I trained was about 18 miles (maybe less, can't remember). So, I froze at around 21st mile mark (remember, I was running at a very fast pace as opposed to training pace during my longest run)...I had to walk the rest of the way. I just couldn't run at all. My legs just wouldn't move. It was a very unreal experience for me, but a very valuable lesson. I equate this to my first ever race when I ran so fast at the beginning that I ended up have cramps in my stomach. I couldn't even breathe. Not an embrassment, but a good lesson. Anyway, two reasons for the wall: I have to respect the distance (as opposed to 5k which is practically nothing) and must run up to 20 to 30, and more miles during training.

Answer 3

It's all related to your mileage and training.

First, most people get stuck around the 6 mile mark in training. So it's no coincidence that 18-20 miles is where they hit the wall because on any given day, you should be able to run 3 times your average daily mileage. (Old runners' rule of thumb).

Second, 18 miles is traditionally a longest run for casual marathoners during training.

Finally, marathoning is a mind over matter situation. So once you get into unknown territory (past 18), and you aren't really trained, then the unknown territory becomes overwhelming. Like driving on a dark road late at night that you haven't driven before. You simply want to find the nearest hotel/motel/restaurant.