I raced motocross up until 2 years ago when I crashed and broke my wrists. I truly believe that motocross is one of the most physically demanding sports out there. You have to have the stamina to ride as hard as you can for 20+ minutes, be strong enough to muscle the bike around the track and be mentally focused to avoid hazards on the track.
I was racing at Washougal Washington one year. It was a huge national that consists of about 3000 amateur riders. Its a little intimating to line up on the starting gate with 39 other riders, looking down the first straightaway into a narrow first corner.
As the gate dropped and everyone accelerated down the start straight I found myself in the middle of the pack entering the first turn. I kept my focus hoped that I didn't get hit from the side. As we all exited the turn still on two wheels I knew I needed to try to pass as many riders as I could early in the race before they spread out. I managed to pass a few guys and within a couple laps I was in the top ten. As I came around one corner I noticed a hay bale in the middle of the track (it got kicked out from another rider). I looked at the bale and told myself "I don't want to hit that." Well.....I ended up hitting it, crashed and separated my shoulder. I was done for the weekend.
It was frustrating I felt awesome during to race. I was passing riders, I didn't feel tired, I didn't make many mistakes. Until I lost focus for a couple seconds and started thinking about what could happen.
If you just focus on avoiding the hazards you will lose your forward momentum and what ever hazard your trying to avoid will get you anyway. Look to where you want to go!!!