Can anyone recommend a backpack that I would be able to simply carry varying amounts of weight in it strictly for running/conditioning. I'm not looking to carry a firearm in it - only stable weight such as sand. I'd like a pack that is reasonably comfortable, won't shift around much, and has a waist belt. I suppose I may be making this more complicated than need be, but am wondering if anyone has any experience with this sort of thing and can make a recommendation. Thanks!