I've hauled packs of rocks around before. I was collecting glacier rocks for landscaping. Rocks are way too dense for training. Just a few cobbles will add a lot of weight to a pack and it probably won't ride right. I used an old military medium ALICE pack and frame, partly because it was cheap, but also because I figured that the military bag would handle the rock without bursting a seam. When I want to train with a pack, I usually carry a bag of dog food or bird seed. Bird seed makes good canoe ballast, but it's actually a bit dense for pack weight. Dog or cat food is fairly dense, but not overly so and a bag of it is somewhat rigid, but conformable. I would start with one or two 10 pound bags and more as I felt I could handle it.