I'm working from home since 15 years and I have nothing of substance to add on how to be productive, stay healthy and be motiviated.
But there is a little thing I find helpful and that is doing push-ups or burpees while waiting for my coffee.
At some point I made a deal with myself - "You want to drink a lot of coffee? Do something healthy and earn it", so instead of waiting next to my Bialetti I do some exercise.
It became a habit and of course I'm doing it only if I'm alone, which is most of the time.
Forming habits is hard, but with a mix of light self-punishment you might be able to trick you into a healthy behaviour.
Also, if I can't go out for a run, I'm using the app "Seven"1 one of many interval training apps that offers 12 exercises to get you heartrate up and keep you fit, without any extras like weights or whatever. Just do those 12 exercises for an intense time span of 30 seconds and you are done after 7 minutes.