I wouldn't lift weights for backpacking unless you are just a very weak person. I really don't think strength is an issue for most backpackers (as long as you aren't trying to carry a huge pack). In my experience of leading kids and adults on pack trips endurance is the key thing. If you look at most long distance backpackers they tend to be very slim and wiry.

I normally find that the new thin guys I take packing tend to way out-walk the big strong muscular types. Your body has to feed and supply oxygen to all of that muscle so the lighter you are means that your systems don't have to work as hard. You also might not have to carry as much food as you don't have to supply a larger amount of muscle mass (although your metabolism might change).

Think of backpacking as more of a marathon or bicycle race instead of a powerlifting event. The fast people that run marathons and do bike races tend to have low body fat and are not big and muscular. I do several thousand miles of riding a year and ride several long distance bike races every year. This keeps my body weight low and I feel like I can backpack all day without wearing out.

Work on endurance and not lifting....my $.02.

-Skippy