Ugh... I feel for ya. Are you going for MOS school or are you on PCS orders? I was stationed there in '01. A buddy of mine just picked up SSgt and is getting PCS'ed there in about 2 weeks. He is going to be an instructor in the school house.
Anyway, you are right down the street from some of the worlds most sought after off roading areas - Johnson Valley. Think Hammers, Wrecking Ball, etc. To the west you have Big Bear which has a ton of trails and to the south in Anza Borrego you have a million and one trails as well.
Shoot me a PM if ya got any SoCal questions or USMC questions!