I raced once when I was 12 or 13. After that, my parents said that there was NO WAY I'd ever do that again so long as I lived under their roof. So I moved out at 18 and took my bike with me.
Turns out that I learned 3 lessons during my first race.
1 - No amount of working out (off of the track) will prepare you for the track itself, except for track time and/or saddle time.
2 - Hold your line! Even if you aren't being passed!
3 - Race your own race. If you start hitting jumps you're not comfortable with, just trying to pass or stay side by side with someone you're racing against, you'll bust it bad, and cross the finishline on a stretcher.