Another thing not mentioned so far. If you're getting a good draft from the rider(s) in front of you, spend some time riding as upright as possible, hands on the tops and don't even try leaning forward. Just be upright and as relaxed as possible. It will save your neck and shoulders getting sore later on, especially if you're thinking of completing the century with the least amount of stopping as possible. When doing century myself, I only stop once or twice to pee or to buy cheap fruits and vegetables from road-side makeshift stalls.
Pros during a long race would also sit upright often by taking their hands off their handlebars. Because their handlebars are very low, taking your hands off is the only way they can sit upright.