If you can find a place similiar to the races your doing then do some practice motos to see how long you can last, then start working on basic lifting and some cardiovascular( running, jump rope, bike, etc.) Then if you get arm pump work on things than used your arms alot, like today i just moved about 2 tons of gravel with a shovel and wheelbarrel. Or if you are closed to a lake and have a boat, go tubing. If i wasd to tube everyday for a month then i would have a really good upper body and my arms would be used to hard stress.
Too make this short, Ride as long as the races your going to at a fast pace, lift lightly for endurance and run, bike, or jump rope. Also some good food is in there somewhere.