I can tell you, your proposed pack weight separates the men from the boys. 15 years ago (45 y/o at the time; 160 lbs; average physical condition), did a High Sierras hike, just below Yosemite, totin’ a 50 lb. pack: 16 miles roundtrip; 4,000 ft. elevation gain; moderate to fairly strenuous terrain; one day in; 3 days at destination; one day out. Despite the fact I was a moderately active weekend backpacker, the pack-weight made the trip less enjoyable. More rest stops than usual.

Afterwards, replaced some gear with new, lighter stuff (sleeping bag, mess kit, stove etc), and took a hard look at my gear list (Do I really need to carry this?). Sliced 15 lbs. off my typical pack weight. It made a difference, and my body appreciated it. Like Hghvlocity said: “Conventional wisdom says a maximum of 1/3 your body weight.” Same school of thought says for long-distance hikes, pack weight shouldn’t exceed 25 percent of body weight.

Maybe you’ve seen some form of this example; probably not totally scientifically accurate, but you get the point: average person covers a mile in 2200 steps; you lift your pack weight every time you lift your foot; ten pounds in the pack is 22,000 lbs. lifted over the course of a mile.

As others suggested, put it to the test. Throw it on your back, and take it for a spin; and, importantly, on terrain comparable to what you’d expect to cover in a “situation.”

The SAS Fitness Training book suggests when starting to hike with a pack, start out with just a bit of weight and gradually increase it as your fitness level improves. This could be a practical exercise for those of us who think we may have to hike a bit. Start a daily walk with a small pack, and gradually increase the weight. That way, you are improving your fitness level and getting real world experience in how much weight you can carry in your pack.

For comparison sake, I look at what I would take on a backpack trip for a week, assuming near total environmental independence (ie I will pack everything I will need to survive except all the water I will use). I've yet to build a pack that weighs less than 50 lbs that will go anywhere I want.

As a comparison, my camelback motherlode weighs in at just over 25 lbs, and contains enough to keep me going for at least three days (including enough water, food, and emergency clothing and shelter). There's of course a big difference between just getting by and living comfortably, which is why the backpack weighs twice as much as the motherlode pack. The nice thing is that my EDC is robust enough and I am skilled enough with it that I don't need to bring the kitchen sink to get by. So really, except for the water, the BOB is not that much value added, more creature comfort.

After some helpful advice from people on this site my Blackhawk Cyclone has a healthy SK, FAK,a couple different shelter options 102oz of water and 3 days of chow, 5 if I stretch it. the whole thing is weighing in at about 40lbs