No repeat ride but a recovery ride the next day on a butt that someone has been kicking for about 4 hours will hurt.

Take it you are doing the ride with a few mates. Remember they are mates and won't mind if you lag a bit. If the pace is a bit too high- Wave goodbye and keep at your pace. Far better to do a 7 hour ride and finish than a 6 hour one and bonk. And talking about bonking- Watch out for around the 70 mile mark. That is the danger time. I normally have a break around the 60 mile mark and eat something just a bit more substantial than I have been munching all through the ride. Only takes a couple of minutes to get off the bike- Eat your meal- stretch and drink a bottle of energy drink.

First sign for me that I am going to bonk is when I do not feel like eating anything. I would have been snacking on Dried fruit- cereal bars- Cake and I normally take a bit of cheese for protein. I also take a couple of gels with me and as soon as I don't feel right I take one. Takes about 20 minutes for it to take effect so slow down for a bit if you feel the need to have one.

Enjoy the ride and get pics. And just remember that this is your first century and you do not have to set a record time on your first 100 miler.