training for 15yr old.

Cardio is one place to start, go jogging every day. increase your distance as your strength increases. Next, hit the track. Have a friend go with you and time you. Do several 30 minute moto's and be sure to push yourself. I know, that sounds like alot but thats what you have to do to win races. Make note of your lap times, you should see them get quicker over time. Do mild wieght training at a gym. Focus on your legs and upper body. One trick to use, take your pulse immediatly after riding. When going to work out, skip rope or do jumping jacks to get your heart rate up to where it would be when you are racing, then do your workout. And most importantly, eat heathly. Stay away from junk food. Hire a dietition to set up a meal plan for you based on your physical needs. As you said, saddle time is the best MX training, and there is no substitute.