Make each rest stop your new goal. Concentrate on your form, and on drinking and eating. Talking with other riders is one way to take your mind off the exertion. Drafting them can give you the physical and mental boost you need to maintain your pace. Don't ride beyond yourself.
Centuries inevitably divide into groups of riders.
Don't blow up trying to stay with a fast pack, especially early on. Let yourself drift back until you're in the company of riders who share your pace. Similarly, don't dawdle. If you feel reined in, ride away by yourself or latch onto a passing pack. It's much easier to ride a century in a group than solo, though for some riders the feeling of accomplishment isn't as great.