I'm hoping I'll be back at work in a week.......... maybe that's ambitious.
RE:- Dodgy knees - replacements only last so long so they like to wait as long as possible. You can only have 2 or 3 joint replacements. My BIL has (in the last 2 years) had a new hip and he's late 30's, so a big problem for him in later life - i.e. how many can he have.
My colleague's husband has had two hips done, and he's a keen hiker, and even still does climbing, and he's in his late 50's.