We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
I see many people doing fairly-aggressive stretching before sports or a weights work-out. Wrong. Except for calf stretches, which are a good idea.
A warm-up for a sport or weights is a 5-minute jog, elliptical, or calisthenics things like jumping jacks. Stretching cold muscle is a bad idea. You can actually hurt yourself that way.
Stretching after a sport, a calisthenics hour, or weights might be a good idea or maybe a waste of time. If you take a calisthenics group program in a gym, you will see that they spend the last 5 minutes or so doing stretches on the floor. That feels good, if nothing else.
Warming up before exercise is always a good idea. I start each exercise of my weightlifting sessions with several reps with an empty bar and slowly move up in weight to my working set.
Stretching, however, is one of those "if it feels good, do it" activities. My squats, deadlifts, bench, overhead press, chin-ups, LTEs, etc., move all my major joints over their full useful range of motion, so I'm as flexible as I ever need to be (wasn't really able to touch my toes until I had been squatting for a while). You'd be hard pressed to find solid evidence that dedicated stretching exercises actually do anything beneficial beyond the temporary "feel good" effect.
Free advice from a stranger on the internet: Probably the best warm up for brisk walking is a few minutes of not-so-brisk walking. Stretching is, I think, especially pointless in this case.
Kudos to your Dad for staying active. My Mom turns 90 this month, just had heart valve replacement surgery last month, was out walking over a mile two days after the procedure. Of course she had been walking that distance and more before the procedure, so that helped a lot. Staying active at that age is critical.
I do stretch, and find that it is pivotal for mobility and avoidance of injuries.
However, I have 2 forms of arthritis - osteoarthritis (wear & tear type), and RA (rheumatoid type). For me, rotating ankles, flexing knee/hip joints - absolutely essential when first getting up from sleep, and for standing/walking after being seated for a while.
I have used 'high-stepping' to get the joints moving smoothly.
However, that has to count as rehab - but, on a daily basis.