This could be a real dumb question but here goes. I read that one reason that people "hit a wall" at around 18-20 miles is because the body only stores about 2000 calories in the form of glycogen and is used up at a rate of about 100 calories/hr. In a marathon does one replenish those calories with energy drinks/bars while they are running so as not to run out or does it take the body too long to convert calories to a useful form?
Thanks.

I've definitely hit the wall out for long runs--gotten all shaky and freezing cold and stranded myself miles from home, but still I try not to over think the when and how much to eat of it.

I usually try to suck down about half a gel pack around the half-way point or shortly after in a marathon. I've found that, for myself, I lose steam at or after the first 18 miles of a run. So I figure, a little sugar-energy around 14 and a couple miles for it to kick in gives me just enough to get through the rest of the race without weighing me down. Plus, the least I can consume the better--eating while running feels gross to me.

My advice would be to keep a gel pack on you on your long training runs and to just pay attention to how your body feels and don't eat it until you notice yourself losing steam. Then, for the marathon, you can try to get those calories in you a few miles before you noticed yourself starting to hit a wall.

One does not run out of calories in a marathon - there are plenty to last many marathons. However, one who is not trained to efficiently use fat as a fuel source will run out of glycogen somewhere from 16 miles on. The way to truly avoid hitting the wall in a marathon is to train a significant amount of mileage at an intensity low enough such that mostly fat is used as a fuel source. One with "decent" fat burning efficiency can make up for some with intake of carbohydrates during the race, but the best way to handle it is through training. You can find some links in the article on low HR training in the "running articles" forum below. There was a time when I couldn't finish a training run of 20 miles at a 9 minute pace without hitting the wall. Now I can run marathon after marathon in the high 6s and low 7s without any carb intake whatsoever.