A couple of cents worth:
Applying specific and cross training will accomplish what you want.
Endurance in the legs and torso can be accomplished with little to no weights:
1. Partial squats with isometric holds (time them and build up hold time to minutes -many minutes). Start with body weight. I know it sounds easy but try squatting down half way and holding it for 10 minutes. Goal is to increase lactic acid threshold and static strength. You can add weight for timed reps by filling empty plastic milk jugs and holding them by your side, even better fill 1/4 with water and hold them in front (not recommended post-shoulder surgery), which will increase arm/shoulder endurance, too.
2. Pick up cheap resistance cables/bands at Academy sports stores. You can do lots of things with them at home, even outside, as long as there's something to hook them to or loop them around. One good exercise is stand on them so that resistance is reached in a half squat position and holding the handles, then power up out of the squat and down. This is power and ballistic training and it's good for the nervous system. (think about how you balance the bike and yourself dynamically when riding over obstacles and bumps: flexed at the knees with your ankles and knees absorbing the up and down movement.)
3. Find something like a trampoline, Sitfit, or anything that you can stand on and practice balance. Start with two feet, squat up and down. Progress to one foot, then have someone throw you a ball (preferably a heavy one; ~ couple pounds). You are recruiting nervous system for reaction and response, and power. Even try squatting on it.
4. Train your core: abdominals and low back. Regular crunches are good, but also include dynamic movements that swing and twist. Watch a Highland Games competition and you'll get the idea, or get one of those big balls on the end of an elastic rope to toss all over the place. Woodchops are the best movement for this (works great for golfing, too), but requires cable stack or ingenuity (anyone figure out how to make a home cable stack, let me know....Hmmm, I think I have an idea....).
5. Upper back and shoulders are important, too. Along with forearms and wrists. Those resistant cables/bands can be used in many ways for that by pulling. Practice good form by keeping shoulders back rather than hunching over too much; too much stress on deltoids and your upper back and back of shoulders will start screaming at you after a few hours. Shoulder presses are good, too. Again, use any weight you can find at home (books, big cans of food, small children
)
6. I can't impress upon riders enough how important core strength is: everything from legs to arms relates to the core. When your core is strong and tight, the rest is loose and ready for reaction. Get a physio (or Swiss) ball; so many things you can do with one at home. From ball squats, push ups, ab crunches, low back hyperextensions and regular extensions, to a stretching aid.
As previous poster commented, if you expect to ride long hours in the heat, devote some time to training in the heat. Best way to help acclimate to exercising and sporting in it.
(Cents derived from my personal training of athletes, and a fellow weight lifter who is also a motocross competitor; we've talked long hours about his training and he's provided feedback on what works and what doesn't.).
Good luck and have fun out there.