Stuffing a backpack with weights is a terrible idea and will only serve to hurt your back and tire you out- what you get out of it isn't nearly worth what you go through. There are special bags that are similar to vests you can wear that you basically fill with sand, but that weight is distributed between your front, back and sides. And really that's just a pain in the ass too. Honestly, if you want to work out just work out. Don't try to pull a Dragonball Z.