I'm sure that such routines exist. There are probably books about them. I haven't investigated because I started exercising when I was younger than ten (swim team) and though I haven't always kept up the routine as much as I`d like, my level of fitness has never descended back to start-from-scratch.

Walking and swimming are both low-impact. I know that for swimming specifically, there are numerous 'Masters' clubs that you can join at any level, where the idea is to keep it up indefinitely as opposed to training for a particular race or event.