Stop at the rest stops, grab some food and drink, hit the porta-potty, and go. 100 miles sounds like a lot, but the only thing that ever bothers me is the saddle. I just want to be off that damn seat.

Some other advice. Try to start at the front of the pack but settle in the middle. Don't overdo the first 10-20 miles, find your comfortable pace there and just stay with it. The most discouraging part of the ride is around 50 miles or so when you are getting tired and realize you are only half finished. I always get a burst of energy at around 70-80 when I see the end in sight.

Overall though if you can do a 50 mile solo ride and not be dead at the end, 100 isn't that hard. If you've trained for it it'll be a piece of cake.