There's a reason people hit "the wall" at mile 20 of a marathon. That's usually about 3 hours in and the glycogen stores dry up. Good reading here:

http://www.marathonandbeyond.com/choices/latta.htm

"
“Hitting The Wall is basically about running out of energy,” says Dave Martin, Ph.D., Emeritus Regent’s Professor of Health Sciences at Georgia State University in Atlanta—chemical energy, that is, stored in the form of adenosine triphosphate (ATP) and obtained from the breakdown, or metabolism, of energy-containing fuel. The runner’s primary fuel sources are carbohydrates (in the form of blood glucose and glycogen, a polymer of glucose stored in the muscles and liver) and fats (free fatty acids in the bloodstream and muscle triglycerides, molecules containing three fatty acids).

Fats might seem to be the logical first choice of fuel for endurance events; not only are they the most concentrated form of food energy, but even the thinnest runners have enough body fat to get them through 600 miles. Alas, it’s not quite that simple. Fatty acid metabolism requires plentiful circulating oxygen, a precious commodity when you’re running at marathon race pace. Carbohydrate metabolism, on the other hand, requires less oxygen. In fact, cells can derive energy from carbohydrates either aerobically (in the presence of oxygen) or anaerobically (in the absence of oxygen).

If you start your marathon at a reasonable pace for you, your fuel consumption ratio will be about 75 percent carbohydrates to 25 percent fatty acids, according to Martin. During the race, as carbohydrate supplies begin to dwindle, that ratio changes as your body begins to rely more heavily on fatty acids.

What does all of this have to do with hitting The Wall? Let’s start with the pace. It’s common, in the excitement of the moment, to start out at a pace that’s too fast for you. Big mistake. Your heart cannot pump enough blood to ensure a steady supply of oxygen to the muscles. At this point, your muscles have no choice but to burn glucose in the absence of oxygen. The anaerobic metabolism of glucose, as it’s called, is inefficient, yielding only about 1/18 as much energy (in the form of ATP) as aerobic metabolism. To make matters worse, among the by-products of the anaerobic metabolism of glucose are lactic acid and hydrogen ions. As these waste products continue to accumulate in the blood and tissue, they will not only make your muscles feel as though they are on fire, but they can also inactivate the enzymes that govern glucose metabolism. You’re toast.

Even if you’re racing at a reasonable pace and you’ve done a good job of carboloading in the days before the marathon, you still have only about 2,000 calories worth of glycogen stored in the muscles and liver; that’s about enough to get you to—surprise!—mile 20. If you manage to deplete your glycogen reserves, say hello to The Wall. As mentioned before, burning fatty acids requires plentiful oxygen, so as fatty acid metabolism increases, your heart must work harder to pump more oxygen-carrying blood to the muscles. It may be difficult or impossible to maintain your pace, especially if you’ve lost enough water through sweat to become even slightly dehydrated (this causes your blood to become thicker and therefore harder to pump). In addition, fatty acid metabolism itself requires glucose; as someone once said, “Fat is burned in a carbohydrate oven.”