Try not to start out too fast. It's SO easy to get caught up in the excitement of the moment ... and there are speedy rabbits that will sprint way ahead, no matter how fast you run! Since this is your first race, it is automatically a
PR (personal record).
I don't know whether you plan to run or run/walk or walk, but even if you plan to run the whole way, I recommend walking through the water stops. You'll get a much better drink that way and it gives your legs a break.
Don't worry about your time, just enjoy it! And tell us how it goes!