Thanks again to everyone for the good tips and advice. I am definately hooked.
I met the goal of finishing the whole race sans injury. Actually finished 17 of 28 in class and 64 overall out of 89 riders. I missed another lap by 40 seconds, but looking at my lap times, that last lap was over 3 minutes longer than any other, so I was pretty spent either way.
I am mighty sore this morning. I've done a lot of mountain biking and the analogy is a good one, but it is more of a full body endurance, not just legs and lungs. I could tell that my concentration was fatiguing as well. I really had to repeat to myself "keep looking down the trail", "don't stare at that stump" etc.
At one point there was a jam on one of the steeper hills and I put my leg down to stop and my calf instantly cramped up. A none too subtle reminder that I have a camelbak on, I should be drinking it.
A little maintenance this week and its off for a second round next Sunday.