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.
These mixtures quench your thirst and are quick sources of carbohydrate as well as some important nutrients. Some riders like to have one bottle of water and one filled with an energy drink. Experiment in training. Some brands may taste and perform better for you than others. Century rest stops often have energy drinks so you can refill your bottle.