Get the blood pumping. 12 mile race should be a really fast and intense one and you don't want to shock your legs. Guys like me would try to get rid of all the guys are not warmed up. Just riding around parking lot is not going to help you. I would warm up at least 30 mins. Oh yaeh, add some intervals.
Everything should be good to go the night b/4. Get there early, so you don't panic and don't have to wait on the line to register. Eat and drink enough, but not too much.
Good luck!