Andrew - those, as you’d suspect, are some pretty broad reaching questions so I’ll try to be concise. Bilateral knee replacement at 50, while not your everyday operation, is done. You’d want to discuss it carefully with your doc as some do them simultaneous, some a couple days apart but during the same hospitalization, etc. and each has positives and negatives. Contemporary polyethylene used in joint replacement is constantly under scrutiny to make it last longer and recent anti-oxident additions have brought claims of 30 year longevity. That’s lab based so far since it’s only been out a few years. You’d of course factor in you expected life span to see what makes sense for you. As for running, were it me, I might run a little. I know a few patients who have and several have said it’s not the same, it doesn’t feel “quite right.” I’d bet, depending on the brand of components used, someone in their company has tested the prosthesis for the increased load and could give you a specific answer. The other thing to consider is that, as you age, you just may not feel like running as much. (More time at the senior center playing bridge!)
As far as mal alignment, the surgeon does his/her best to avoid that.
John