I've done quite a few multiple day backpacking trips. On most of those I'm setting up and tearing down camp everyday and averaging 10-12 miles of hiking in mountainous terrain with all of my gear on my back. I try to stay around the 40lb mark which includes three or four quarts of water in the bag. I'm 6'1" and 145lbs so I have a skinny frame.
I carried 51 lbs during a five day solo trip on Isle Royale and would have been fine except I twisted my knee on day two when a rock I stepped on moved. Fortunately the terrain is fairly flat there, I had brought a few pain killers and was able to break up my daily hikes by stopping to fish different lakes. Pan fried morel mushrooms and northern pike fillets gave me the motivation to keep pushing and kept a smile on my face.
I did a three day trip in the Sierras this summer with a 35lb pack including crampons and my ice axe. We covered 10 miles each day but on our climb day we left camp set up and carried day packs. Banner Peak elevation 12,936', my highest so far but I plan to climb Mt. Whitney next spring.
I have a tendency to overpack as far as food and clothes but dry clothes and a chocolate bar can really lift your spirits after a full day of walking in [bleep] weather.
SPEND THE MONEY AND GET QUALITY BOOTS!!!
Overall the best thing you can do is pack your bag, put it on and go for a hike. You'll need the physical training and you'll get used to weight while learning to pack a balanced load and adjust the straps to properly fit you.
