give way to those faster than you, don't block them. if someone is on your wheel and they are clearly stronger, pull over and let them by.
if you don't intend to win, don't start in the front, but don't start in the back either, start just behind the halfway point in the pack.

if you get a chance, pre-ride the course and see where you are gonna need to climb, see where techy parts are, in general, prepare.

other than that, as trite and cliche as it may be, relax and have fun. treat it like a big fast group ride, for you, that's all it is. you may be surprised and do better than you think you will. if you are riding centuries, you are strong enough for 12 miles. the only thing that may hold you back is lack of mtb skills, depending on how techy the course is and how skilled you are (i have zero knowledge of both factors so i can't say). assuming it's like a standard cross country race, it will be little more than a road race and you will do fine.

seriously, relax and have fun. and let us know how it went, i wanna hear what it was like.