I don't have one, but nor do I have a car. So the secret is: walk everywhere. Walk walk walk.

That keeps me 'fit'. I'm 49 and perfectly healthy, good weight, 120/80 bp.

I know I need to do more strength training but I'm not in a place (spatially) where I can do that.

Diet is of course a huge factor, but you didn't ask about that so while I'll allude to the can of worms, I shan't open it.

I was thinking to start planking (not just for fitness but also as a mean for stress inoculation).

1. You can do it at home. 2. It's short: when starting out, you might just be capable of holding the position max 1 minute. 3. you don't need special dressing or tools, the floor is all you need (a yoga mat would make it nicer though) 4. when done on the elbows, you can even use your phone while you do it (provides you consider that healthy)

It really seems a no brainer form my angle.

I am also getting interest in Fascia training (that's for old men, mind you)