Regarding knee replacement, I had both of mine done 8 months apart, almost 6 years ago now. The first one took a year and a half to feel normal and the second one took about a year. Once you get past the initial seemingly-fast improvements, it starts to feel like it's going to hurt FOREVER and just isn't getting any better. Trust me - it's improving a little bit at a time and you just have to be patient. Keep moving as much as you can without doing too much. Walking is the best thing. I found recumbant bikes were better than the regular upright bikes. If you can, maybe join a gym that has those and stair-steppers. Here in Florida stairs are few and far between and I could tell I needed to do them to exercise my knees and the stair-steppers helped.
As for points, when I worked I was on the road a lot and played the game to the hilt. That was when you could get status on Delta by flying segments so I'd come up with some creative routes to get where I was going. At that time Delta was the only airline flying out of our local airport and since I wasn't paying the fare I didn't care that it cost so much more than driving an hour to the big hub airport. Now that I'm retired I'm actually finding that I'd rather just use a cash-back card. It gives me more flexibility to choose the airline, hotel, and whatever else I want. Plus the last few years I've had to cancel several trips for various reasons (not just Covid) and it's a lot easier to do if you didn't pay with points. I don't like having to book through some credit card's portal that doesn't offer all of the airlines, or have to stay at some particular chain hotel when others might be a better choice (and I like to go to out of the way places that don't even have chain hotels). YMMV but for me the simplicity has become more appealing than playing the game.
Having said all that, I DO watch for sign-up bonuses and will collect and use the points from those and then cancel the cards. I have a hard time justifying paying an annual fee when I don't travel all that much.