Ride, ride, ride. Nothing gets you in shape like riding a dirtbike. It also helps to hit the gym and focus on endurance. Medium weights, more reps. I used to go with light weights and lots of reps, but found the added strength of using a medium weight (not the maximum I can lift) and as many reps as I can handle works really well. I also ride a bicycle to and from work (12km each way, 6-10% grade up hill on the way home) which really, really helps my stamina. I lift weights 3 days a week, ride the bicycle 3-5 days a week then also ride my dirtbike 3-5 days a week (minimum 2 hours per ride - weekends I try to get as much riding in as possible).
I also recommend hydrating before the race - I try to start on the Wednesday before a race. Have a big pasta dinner the night before the race, then I try to eat a big breakfast in the morning (though some people can't eat a thing due to nerves). Bananas are really good to help prevent cramping. For a 3 hour race, get a camelback or similar drink system. Some people use water, others use gatorade but I've found Eload works the best. No cramping, just lots of energy.
For the actual racing, remember to pace yourself. You can't win the race in the first 30 minutes, but you sure can lose it. Ride your own race and you'll do just fine.