Most people think motocross requires little, to no fitness. This is not the case at all. Motocross is the second most physically demanding sport besides soccer. Being an avid motocross racer myself, i know how much you have to train just to get your fitness up to par. If you want to be a competative racer you need to spend almost as much time on a bicyle and in a gym, as you do on the actual motorcycle.
Some important things to work on is,
1. Strechting
2.Abs
3. Cardio
4.Legs
5.Back
If you follow these tips you can help improve your endurance on the bike, and even if you're not a racer these tips can help get you into the best shape of your life.