As so many runners say, don't try anything new on race day!
Make sure you've worn your shoes for at least four long runs. Dress in layers--get some throw-away clothes from a thrift store to toss (at an aid station) after you've warmed up! Hydrate well in the few days before the race, and stick to the nutrition you used in training!
Most of all, enjoy the experience! You are doing what so many people can't!