Bench press and other weight machines mean nothing to me. The best exercises you can possibly do are against your OWN body weight.
ie. push-ups, pull-ups, sit-ups, etc...
When I did... It was for high school wrestling some time ago. And I put put up about 125 lbs. Didn't dare go any more... I usually stayed around 95 lbs to get a decent workout. I wasn't looking to max out... If you get a decent daily workout your plenty strong, and you don't need to go tearing your muscles more with that added stress.
Backpacking... I have no idea. I've never felt the need to weigh my backpacks...