Tip1. Don't be intimidated by the distance. Further is much easier than faster, a century is easy if you ride at your own pace.
Tip 2. Start slow, finish fast. If you go out too fast, the last twenty miles will be very hard. If you're feeling good at 60-70 miles you can always speed up.
Tip 3. Hydration is much more important than food. I can ride a century on virtually no food, and often ride upwards of 70 miles on water alone. But until you know your own body, and how it reacts to distance, it's wise to eat a little, and often.
Tip 4. Don't change your set-up, or wear new clothing, use a new saddle etc. for a century. Being comfortable is the most important thing.