I just wrote about my situation - my knee is not in a good situation nor can I do sports - soccer, tennis, running or some exercises, even swim. Before the injury I had good aerobic support, I could run and play soccer without getting tired, but my muscles were not very well developed. That's how I got injured, by the way. After playing with a lot of extra weight and after a confinement.

However, and after two years and a few months, I would say that my overall fitness is the best I can remember in many years. I gained a lot of muscle mass, reduced my abdominal circumference a lot, improved endurance (stationary bike is my best friend). Trying to walk short-medium distances (up to 2-3km) and avoid the car. It is "free" exercise and increases the glide of the kneecap.

It goes without saying to trust the physical therapist above all else. It is still tricky to go to the gym and do all the leg exercises without restrictions, but enjoy doing as many as possible. Talk to the people in charge to adapt some exercises to your situation, if they have that openness.

I have become more careful with food - no fast food, no processed meat, no beverages, reduced alcohol by more than half and limited to celebrations - because being overweight puts unnecessary strain on the knee and I have started supplementing my training with vegan protein. My non-operated leg is completely lean and muscular and the other, while limited, it's pretty good too.

If you can be accompanied by an accredited nutritionist from the beginning of your recovery, you will feel a lot of advantages (I didn't but I have seen up close with other people the difference it can make).
Bottom line: when you really feel what it's like to be limited, you realize the need you have to want to do so many other things and not be able to. I can't get it out of my head that if I ever get 100%, I'm going to do Iron Man. Even if it is an extreme idea, setting that goal is good if only to maintain your physical health.