Good for you for getting out there and doing it!!!
I play on my mt bike, I'm more of a roadie, but I can give you some general tips. This is how I train for a century:
Progressive overload - do a little more each week and be sure to recover between sessions. If hills are a problem, train hills and make sure that you give yourself recovery time in between. The time between sessions is when your muscles, tendons, ligaments repair & get stronger. If aerobic fitness is a problem, do intervals. Spinning classes are great interval training. Don't do more than two sessions of interval training each week and be sure to space them apart.
The week before, taper off. The last three to five days should be light to no-riding and carb loading. Especially the night before, be sure to eat something high carb and easy on the stomach. Not too much, or you won't sleep.
Race day you'll find the adrenalin helps you get over the hills & scream down the trails.
Most important tip = HAVE FUN!