Good advice by Jason!
I just rode my first race two weeks ago and I'll be doing my second on Sunday.
Just like Jason mentioned, I rode pretty hard on the first lap and half way through the race I was blown.
Didn't help that it was sunny and 75 degrees!
One thing that I think slowed me down was the fact that a lot of the race was on grass and my tires may have been at too high PSI for those conditions. Riding the grass was like riding over a giant washboard. My headset had pretty much rattled loose by the end.
One piece of advice I can add from experience would be to note that tire pressure really is important! Set it as low as you think you can get away with without getting a pinch flat.