In retrospect, it seems that when I hit 65, it seemed old to me and I kind of gave up. I didn't realize it at the time, but I think that's what happened. Over the next year, I loafed, ate, drank more wine than I should, and put on 25 pounds.
The thing that saved me was my wife. She didn't nag me or anything, although she would be encouraging if I seemed to want to do anything healthy, like go bike riding. But she has always been in great shape, physically, and spends 2-3 months a year in the mountains backpacking. I join her when work allows, but frankly wouldn't want to spend that much time in the mountains even if I could. But I did plan my normal two weeks to backpack with her on part of one of her extended trips, which we spent mainly off-trail above timberline, 10,000-12,000' in the high Sierras.
The first few days reminded me of Marine Corps bootcamp, but without all the yelling. But I hurt and suffered just as much. Then I gradually began feeling better, so by the end of the trip I was 15 pounds lighter and felt 10 years younger. My attitude has improved so much since the trip that I have lost the additional 10 pounds I needed to lose, and am back on a good exercise program. I really hope I can make it stick, although I know from the past that sometimes it does, sometimes it doesn't. But I think at least I've gotten rid of feeling sorry for myself for getting so old, which was a big part of letting myself get so badly out of shape.
Going on a two or more week backpacking trip has always been good for my attitude and my health, and I can always count on losing about a pound a day, eating as much as I want to eat of pretty basic food without any frills. This isn't loafing around camp, of course. We cover 6-7 miles a day, and usually gain and lose 2,000' or so in elevation every day, but even so, we are usually at our next camp by about 2:00 PM, plenty of time to set up camp, loaf, fish, whatever.