Ouff, I'm sorry about your knees.. I don't know if biking is any better either then? Otherwise, as others have said, it can be nice to just sit outside and read a book, or just lie down, look at the sky, and listen to something for an hour.

It's just that being inside in one place all the time, constantly looking at the same screen, is bad for us. Evolution has built us for more varied stimulus than that, and pretty much anything that breaks it up is probably better than nothing. Wish you well :)