I'm doing my first ever mountain bike race next weekend (first race ever also..don't road ride)

My question is that it is also a 12 hour race and I will be doing the 3 hour race which won't start till later that day, but due to the fact that my BF will be doing the 12 hour I gotta hang around all day till my time to race.

I am wondering what I should be doing all day while waiting. What should I be eating, Should I do a few short rides just to warm up during the day. Or will that wear me out?

THANKS...

NEW RIDER,
CHRISTINE