You are going to find that you are in better shape than most of your competition. Learn how to stand and carry speed in the corners and you should dominate your class. In my area, the guys that dominate are all either mtn bike competitors or have do triathalons. For starting riders, their biggest issue is almost always having enough fitness to finish an event. Once their fitness level improves, they can work on their riding technique. When you get back home, set up your suspension, go out and have some fun.