At 69, I can see the changes happening. A year or so ago, I asked myself where I wanted to be 10 or 15 years from now, and the answer was "still backpacking." Having spent most of my life with "responsibilities" such as a family, mortgage, college, etc., I hadn't been extremely athletic, to put it mildly. Realistically, there's no real possibility that, as I retire (semi-retired now, fully retired in another 18 months), I'll suddenly morph into some physically enhanced version of myself. What I have will just have to last. (I'm not in horrible shape, but when I get up in the morning, everything works and nothing hurts - and it's all original equipment.)
The first big change came from realizing that my knees, which are in really good shape, are getting older and can't take the pounding they once could. So, I decided to lighten my load by losing another 10 pounds from me (I'm at 6 so far) and 10 from my pack. Mostly by figuring out what I didn't need, and by some sage gear-replacement advice from my Gearaholics 12-step sponsor (who also happens to own the local backpacking shop), I'm now carrying about 16 pounds, all in, on my weekend trips.
About those trips. I've reluctantly concluded that a three-night trip will be about my limit - mostly, that's about the longest chunk of time I seem able to snatch away from other demands (such as a 92-year-old mother nearby in assisted living) and things that my wife and I have put off too long. The up side is, the food weight doesn't become a crushing burden, and I get to go out more often. (I'm now getting out twice a month for two nights, usually in the middle of the week - avoiding weekend crowds is another unexpected retirement benefit.) To get the most out of those trips, I've also decided my destination must be no more than 4 hours away - which leaves me a lot of good hiking in Ohio, Kentucky, and Indiana. No, it's not the heavenly hiking those of you in the West enjoy, but since I've never gotten the midwestern woods out of my system, it's enough for me.
I've also decided that heavy rain, snow, below-freezing forecasts, and other "been there, done that, got the T-shirt" trips are out. (I have plenty of T-shirts, all in good shape.) Again, not having to carry "mission-specific" gear lightens the load - but eliminating those conditions also reduces the chance of a non-lethal knee or back injury that could end my backpacking career. (Nothing like sliding down a mud-slicked, non-switchbacked 30-degree slope to get the old heart racing!)
I've also eliminated trips to places named "Mountain" or "Highland." My stamina is not what it was at thirty, and I no longer can take on several days of large (for me) elevation gain and loss like I'd find on the AT. I've also shortened my hiking day to 8 or 10 miles. These changes mean I'm not exhausted at the end of the day. The side benefit is that I walk slower, and see more (or more deeply.)
This also means I don't go on as many group trips as I used to (and my days of leading beginner groups are over.) I can't keep up with the increasingly-younger demographics of those groups. As compensation, I now hike with one or two people my own age that I know well and who share my limitations. We find a lot of pleasure talking and in observing the natural world more carefully than we used to (amazing how much we missed on the same trails, when we were younger.) I'm finding I'm having more fun, sleeping better, and grinning more as a result of limiting the types of trips I take.
Luckily, in Dayton, Ohio, we're blessed with a nice regional system of parks and trails. Within 20 minutes of my house, there's a 35-mile backpacking trail, complete with elevation changes, old-growth forest, and backcountry camping, that I can day-hike to my heart's content, and get an overnight permit for with an hour's notice. And my buddy and I usually have it all to ourselves. Within my 4-hour limit are half a dozen state and national forests with 20 or more (often many more) miles of trail each, plus Kentucky's Red River Gorge and Sheltowee Trace - they're not the Wonderland Trail or the Winds, but for what I can and want to do, they're hiking heaven.
And perhaps that's the greatest adjustment of all: finally figuring out what my own personal hiking heaven looks like, and then finding it on my doorstep.
I'll let you know how my plan worked out in 2030.