My mum has had one hip and one knee replaced, in her 80's. She bounced back pretty well from the hip but the knee took longer to recover from. It was more painful, particularly at the front of the thigh muscle, which she said they 'stretch' during the op to make it all work, though I don't know if she's right about that.
Persistence with physio etc helped and the pain abated after a couple of weeks and she was rid of the awful knee pain, too. Her other knee has gone now but she doesn't think she'll have it replaced because it's as painful as the first one and she feels that at almost 89, she won't really get the benefit as when she was a bit younger because widespread arthritis has limited her general mobility.
I hope you soon find your knee improves a lot!