for now, i would say just have fun. don't want to mentally burn yourself out on mtbing. to specifically prepare for mtb races though, i would suggest going to sourlands, getting out of the car, getting ready, and just charging straight up the fireroad climb. course designers tend to like to start you out on climbs. it's somewhat for the better though, because it seperates the contenders from the people just out to have a good time, and tends to get everyone singlefile before the singletrack starts. after charging up the climb, keep pedaling. if you reallly want a good position going into the singletrack, you need to be able to beat a good amount of guys up that first climb, and still have the energy to keep them behind you.
also try doing a few rides non-stop. the stops on group rides aren't there in races. granted, j likes to keep you moving, but....
