give way to those faster than you, don't block them.
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.

other than that, as trite and cliche as it may be, relax and have fun. 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.