I'm 63 and still backpackin' and still collecting the latest backpacking gear. It's a chronic sickness (the gear buying) but when I'm on the trail with 10 or 15 fewer pounds than I had in 1980 I'm a very happy camper.
Keep working out, taking conditioning hikes, taking Glucosamine/Chondritin tablets and the occassional Ibuprofen. You'll be able to do this 'til you're 80 or more. [color:"green"] [/color]
"There are no comfortable backpacks. Some are just less uncomfortable than others."