Exercise is vitally important as we get older--"use it or lose it" is the key. It's not just backpacking but your present and future health that depends on it! Try a brisk walk at noon for half an hour (or exercise at a gym if your work is near one) and a brown bag "working lunch" at your desk. (Good way to cut calories and expenses, too, by avoiding restaurant lunches.) Now that the days are longer you can do another brisk half-hour in the evenings. There is no rule that you have to do all your exercising at one stretch; I've seen a number of articles that claim splitting your exercise into two or more sessions is just as effective if not more so. You could get an exercise bike and bike through the evening news (the news is easier to bear if you are working up some endorphins via exercise!). The bike will exercise your hip and knee joints with much less stress (increase the bike's resistance very slowly). You could combine the exercise bike and weight lifting (or other "core" body exercise), alternating days for each. These are all things I tried to do when I was working long hours. It really helps if you can persuade some of your colleagues to walk with you, but go alone if you can't.

You do need to work up gradually to trips like the one you took; even if you're exercising daily, that is a pretty strenuous effort you just did! I suspect that at least part of the hip problem is that your joints need good strong musculature to support them, and if you haven't been exercising regularly it's the joints themselves, rather than the muscles, that get the wear and tear. So it's really a good thing that you bailed.

At 72 I still go out backpacking a lot, thanks mostly to this site for helping me cut my load by more than half. My trips are a lot less strenuous than they used to be--maybe 5-6 miles a day. While I'm slower, I enjoy the trips more because I'm looking around at my surroundings a lot more instead of putting one foot in front of another. I also do more of the backpack in/set up basecamp/dayhike a couple of days/backpack out sort of outing rather than trying to go long distances with a full pack every day.

As you mentioned, altitude was undoubtedly a factor. While I acclimatize relatively fast, having grown up in Wyoming --home at 7000 ft and spending summers up at 10,000-11,000 ft., I still try to take it easy the first few days at higher altitude. While I acclimatize rapidly, my hiking buddy--my dog--was raised and lives at sea level, so he needs more time to acclimatize (or at least that's as good an excuse as any!),