Walking down mountains is always a mistake.
Jan and I walked up Ben Nevis (and down again) when we were in our early 50s. Ben Nevis is actually quite boring, and the tourist route is very badly eroded, but going up is all up, with no variation, and of course coming back down is all down, with equally little variation. The knee cartilage gets badly punished by such things, and it did take two or three days before the pain and discomfort subsided. Just before we got back into Fort William, a young couple who had also climbed the mountain, overtook us. We had a brief chat. "I think it's wonderful that you can still do it at your age!" he remarked. They still haven't found his body...
I have had a lifelong tendency to obesity, but I have always tried to keep myself active. Up to May last year, I had kept up a daily average of >11,000 steps for about 18 months. Then my brother died relatively suddenly, after having had 76 years of being very fit, and not long afterwards a whole load of other unrelated family trauma overwhelmed us and I responded by comfort eating. I have ballooned in weight again, and I became much less active, but I'm beginning to get on top of things (I hope) and I'm trying to cycle at least 10k each day. I also attend a gym for an hour weekly (see
Wowbagger's Weightlifting for details) supervised by a guy who is a physiotherapist, at a sports injury clinic. I look around at other people my age (I'm in my 70th year) and I don't think I'm doing too badly. If I could shift 20kg I'd say, just like that other noted son of Billericay, that I'm doing... very well.