It would be hard to find a cheap frameless backpack that would comfortably carry 30-40 lbs. I think the injury risk is overreacting for training. If it becomes uncomfortable, take it off. It is good to do some training with weight. Start with what you got and see how it goes. You can often find old external framed packs at St. Vinney's

*edit: forgot to add.... Just get some weight on your shoulders and head to the Kettles or Holy Hill and see how you do. It is definitely harder hiking up hills with extra weight, but you are not going that far. If you are in shape you shouldn't have too much of a problem. Just slow down and take your time.