Just because it is too heavy? What do you mean? Is it overtraining to do 75 for practice but then carry less when you go out on the trail?
If you want to train with a 75 pound load, get a pack that will support such a load properly. A pack with a frame carries much differently from a pack without a frame. The area around your hips should be 'hurting', not your shoulders.
I would not consider carrying a >10 pound load any serious amount of time without a proper backpack (meaning frame and hip belt).