With about 1500 other riders doing STP in one day, you'll have no trouble finding pacelines. Take it easy at first, don't try to jump in the fastest groups or you'll be worn out before Centralia. Go at your own steady pace for the first couple of hours. By that point, you'll be in the group of riders matching your own natural pace so hop in a paceline if you feel like it. Don't spend too much time at the rest stops. Just eat, pee, refill the water bottles, and go. If you're on the tail end of riders, note that much of the food will be gone by the time you arrive, so plan accordingly (there are a number of gas stations/convenience stores along the route if you get too hungry). As Homeyba said, STP is mostly flat and a lot of fun.