Hi Mull, sounds as if your knees are worse than mine were, but I didn't have your heart problems to complicate matters.
I had knee pain for decades - I think it's genetic, my younger daughter's knees click when she stands up, just like mine used to at her age. My main hobby and exercise was weekend hill walking, and about 10 years back the pain was getting steadily worse so that I wasn't able to do the distances with my then other half (the knees were a part in the breakup of that 18 year relationship).
I had to spend a year going through the NHS physio hoops to get to an MRI and a consultant surgeon who told me that the cartilage was damaged beyond any repair that arthroscopy could do. He said "keep taking the pain killers and come to see me again when you decide that you need a new knee". I carried on for another six or seven years, with steady degeneration until one day I was out with a group in the Lakes, doing well until we started to come down.... that night I couldn't even stand up for a pee, so I decided that it was time. As I had already been through the NHS preliminary route I was able to go to my GP and ask for a referral to a surgeon. I chose one who only does knees, and who was recommended. First appointment with him was blunt - I said that I could still walk five miles and his response was "you don't need a new knee then - change your lifestyle!"
Thing is that the outdoors is a major part of my chosen lifestyle - I live next to the Peak District, and when the five miles became three a year later I went back and said that I wanted the op. He huffed but agreed, and the worse one (right) was done in 2019.
Op was under epidural with mild sedation. A few months beforehand, channel 5 had broadcast an Operation Live episode on knee replacement, which i had viewed. Part way through the op I got bored with the soft rock through the headphones so removed them to listen to the conversation down at the other end. Couldn't see anything because of a sheet hung in the way, but pretty sure that I have a #4-and-a-half fitted...
Recovery took a few months, painful for the first 2 or 3 weeks but then improving. I had a great physio (seeing her again now with the second one) who encouraged me to push the rehab. 3 weeks after the op I was kindly given a lift over to Cranage and spent the day wandering round the hifi show on crutches. 6 week post-op I could drive again, and did a 6.5 mile walk 10 weeks post-op. Since then it has given little pain - OK, it's not as good as a real one in good nick (limited bend, and no crawling allowed) but it lets me do a lot more than I could previously.
Left knee this year was with a different surgeon (don't know why, but another knee-only man). Epidural again, but this time the sedation knocked me right out through the op (I suspect that his technique is a bit more heavy handed than the first chap's). They were happy for me to leave hospital the same day, but I opted to stay in overnight and was out the next morning. The dressing after this op is a lot neater than the first, no drains and I left hospital with just a transparent sticky strip over a neatly stitched wound. Recovery so far going along the same lines as last time.
Sorry everyone else for the lengthy exposition, hope your genetics and life mean that you don't need this work (but if you do then my experience so far is that it's worthwhile).