The best way to get in shape for carrying heavy loads on your back in the hills is to carry heavy loads on your back in the hills.
3 1-gal jugs of water in a pack is 25lb. When I'm working my llamas to get them in shape, I load the panniers with water jugs and carry a pack myself. By hunting season, I have some well traveled water and both the llamas and me are much better off, too.
