Stay fit and maintain lean muscle mass. You don't have to be a crazy fitness nutcase, just try to walk a few miles a day and do some kind of exercise to get your heart rate up 3 times a week or so. Try not to eat total crap, but honestly this matters a lot less than the first part. Be thankful that diminishing returns kick in really early here.
I'm in my mid-50s now. My friends and I are all getting old. Each year the chasm of difference in lifestyles between my fit friends and my unfit friends grows larger. At my age it's starting to feel bleak and grim. The older you get the more your fitness is going to affect your quality of life and happiness. In your 60s it's going to determine whether or not you are even mobile without assistance. Aging ain't fun for anyone, but it's fucking brutal and painful if you're out of shape and/or overweight.
The older you are, the harder it is get in shape. But it doesn't really get any harder to stay in shape once you're there. Don't put it off. If you don't move you're building towards a future where you can't.
I think you're really overstating things here. I've lived all across the spectrum from dense urban to acerage rural and I think it's a lot more feasible than you're depicting to go for a walk around the block in a wide variety of living spaces.