Some riders like to eat every 20 miles, some prefer every 10, others go by time. You should have discovered what works for you during your long training rides. Don't pig out at the rest stops. It's better to munch lightly and steadily instead of stuffing yourself. Large amounts of food divert blood from your working muscles to your stomach, which can weaken and even nauseate you.
Fluid is as important as food.
Hydrate yourself well by drinking extra water during the two days prior to the ride, then have 3 to 4 glasses during the morning. Once you're on the bike, a good general rule is to empty at least one standard-size bottle each hour. If you have trouble remembering to drink enough, set your watch to beep every 15 minutes, signaling sip time, or drink whenever you notice a cyclist reaching for his or her bottle. This will help you avoid dehydration and the fatigue it causes. Energy drinks are a good substitute for water and food.