I can attest to the advice many have given regarding pacing and hydration. My first century I didn't do either very well. I finished, but at the 75 mile mark, I was really hurting. If you start out slow and find you have lots of spare energy at the 75 mile mark, you can always pick up your pace then. You'll learn a lot riding your first century and will apply the lessons learned to subsequent rides.
The other thing I would note is to pay attention to the weather forecast. On my second century, I did better on pacing and hydration but the temperatures went deep into the 90's which, for me, is way too hot. The heat really got to me. As with my first century, I finished, but was hurting. I won't do a long ride in that kind of heat again!
On another ride, the weather forecast was for the sun to break through and a light morning rain to stop early in the A.M. It didn't, and the ride (ended up just doing a metric century) was cold, wet, and raw.
Take note of the weather and where your comfort zone is, what for you, may be the extremes of heat or cold.