A couple additional things occured to me at lunch. Try to stay loose and stretched. I'm a yoga devotee and there are a handful of hamstring and hip stretches that I do before every ride, but especially before intense/long rides. If you have a stretching routine, I would suggest doing it the night and morning before the ride. Repeat the stretches as needed at rest stops.
Also, keep an eye on tension in your upper body. Especially at the start, you may find yourself clinching your shoulders or locking your elbows. Try to stay relaxed instead.

You'll notice on a century (or longer) that your body and mind will go through various stages, some better than others. Short of bonking or heat exhaustion, don't be surprised if you have peaks and valleys in your energy level, and there may be no rhyme or reason to them.. Try to just ride through the worst of it and try to hold yourself back a little when you're feeling really good. You may need that excess energy later.

Again, good luck!