I can make it through even a half-marathon on a good breakfast and carefully-judged pre-race hydration, but to go much longer I need more food (and water) mid-run. The long-term exercise involved in a long race is fueled by a combination of fat reserves and glycogen stored in the liver and muscle tissue. Glycogen is the more efficient fuel, so as exercise intensity increases, muscles draw on it more heavily.
For far more detail on evidence-based endurance training approaches, I suggest Dave Munger's great science-based running. See you in 26.2 miles! ◼