If you're going to backpack, work on your legs, core, lower back, and hips as much as your upper body. Do exercises that work as many of those at once as you can, e.g. do back squats and front squats that work your hips, lower back, and core instead of a leg press machine that just works the legs.
If you have access to a good trainer, hire him for a couple sessions and explain exactly what you want out of it--to be able to carry heavy loads for miles in the mtns. Have him show you different exercises, then do them on your own. And concentrate on the stuff that challenges you the most.
