For your endurance training, slowly build up your distance much like you would for a marathon but train on trails, not roads. If you prepare just on a treadmill, you won't be prepared for the terrain. It's important to train on trails, say, once a week at first, but do it on a short medium intensity day just to get your body used to those higher impact forces.
Like marathon training, you should incorporate short days and long training days each week. Also, be sure to adopt a strength training routine. If you fatigue during the race at the hips, trunk, or shoulders, your body will be more likely to break down and the last thing you want is aches, pains, or worse.-- Paul Robbins, performance specialist for Athletes' Performance