Hayley, Cascade has a suggested mileage plan for training for STP, right here:

http://www.cascadebicycleclub.com/Ea...tp_mileage.cfm

You don't have to follow it exactly but it doesn't hurt to try to get close. If you train, you'll be fine and it really can be lots of fun. The other recommendation I have is, leave EARLY, as early as you can. It helps keep you out of the big crowd at least at the beginning, before people start spreading out.