Do you have a bike, or are you borrowing one from a friend? If you have one and you haven't used it in a few years you may find its not in good working order. Dig it out, wipe+oil the chain, pump the tires, and take it for a test ride. Make sure the brakes and gears are working. Go for a 5 or 10 mile ride - whatever you have time for before the weekend. Make sure nothing obviously feels wrong about the bike. If borrowing, I'd see if I could ride the bike ahead of the weekend if possible to test it out.
Other ideas are just make sure your friends realize you may be slow and want to stop more often than they do, and that your legs, butt, and/or other body parts if the bike doesn't fit well, may be sore when you are done.