The Wall
If you ask most marathoners about the marathon, they'll probably tell you that a marathon is halfway over around the 20 mile mark -- the last 6.2 miles are just as tough as the first 20. This seems kind of odd to most people; you're almost done the race!
The reason the last 6 miles are so difficult is that by the end of 20 miles, you're going to be mostly spent. You've used up all your glycogen, so you begin burning fat instead. Your legs feel like absolute crap. You stop thinking straight (at least less than before).
It's misery.
Fortunately, it's also avoidable. Many runners never hit the wall. The secret is twofold, and both parts rely on good preparation.
Eat More Pasta
The entire reason you ``hit the wall'' is that you've run out of glycogen. Most people have about 2000 calories worth of glycogen stored in their body. Knowing that each mile of running burns around 100 calories (regardless of how fast you're running), that tells you you should run out of glycogen around... Mile 20! Surprise!
To get enough glycogen, the first step is to carboload in the days before the marathon. People used to try to deplete their bodies of carbohydrates before the marathon (3-5 days before). The body gets used to not having enough glycogen and tries to store all that it can. Then, you take in tons of carbohydrates in the last days before the marathon, and your body stores as much as it possibly can.
Recent research has shown that this depletion and loading isn't necessary. As you taper for the marathon, just eat plenty of pasta, and be sure to take in lots of water (you'll need it when you sweat for 26 miles).
Right before the marathon, take in a last bit of carbohydrates -- two or three hours before the race begins. During the race, drink fluids -- water or sports drink (for an extra bit of carbohydrate during the race).
Long Runs
The other critical component to avoid hitting the wall is proper training, and the biggest part of that is high-quality long runs. Read the notes about long runs for more information about why they're critical for success in the marathon.
From a ``hitting the wall'' perspective, long runs are an essential component of success. When you do a long run, you teach your body how to take advantage of fat reserves more effectively so that decreased glycogen won't have such a detrimental effect.
Just as important, when you do a long run, you train your body to store more glycogen in the muscles. And of course, the end result of this is that you put off the depletion of glycogen, avoiding ``the wall.''