Hi,
I read this post this morning, and was mulling over what I would do. I race mt bikes too, so I am a dirt sister.

Now Part One:
Move up...you are young and have many years yet to race and to win. Scope those girls out this season and take 'em down next year. ( can you tell how competitive I am?)

Part Two:
I would be spending most of my time off road. Now in my opinion, as a fellow mt biker... I gotta say, girlfriend, it is gonna be ugly. The 42 miler is the only thing that made me hesitate to scream MOVE UP when I read this post early this morning.
You have to focus on just finishing that puppy. Start adding in a longer ride once or twice a week. Don't worry too much about tearing it up, just pace yourself. Work on your nutrition too. Hydration and nutrition are going to be key.

I hope you plan to keep us posted. I will be anxious to hear how your training goes.