I might suggest you get books on training and conditioning for endurance. I would also go to the AERC web site and see what info they have available.
As for cross country...are you talking about trail riding and hopping over logs and stuff, or are you referring to the events where the horses go over jumps that will not give, into ponds, and over things that make my hair stand on end?
That sort of thing takes years of training for both horse and rider and an extreme amount of talent for both. And a boat-load of money to get the horse and rider to the point of competition.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger