In an endurance event, your energy comes from a combination of stored glycogen (from carbs) and fat. Even the leanest rider has enough fat for 100 miles, but your glycogen stores are limited to a few hours of hard riding.
Three days before the century increase the carbs you eat, while cutting back a bit on protein and fat to be sure that your glycogen stores are full. Carbs aren't just pasta, potatoes, bread and rice. Vegetables and fruit are also 100-percent carbohydrate. You'll probably gain a few pounds because glycogen is stored with water. Don't worry—you'll need the water for the event.
In the days leading up to the century, drink plenty of primarily clear, unsweetened liquid and avoid alcohol. If you're drinking enough you should urinate with a good clear stream.