I had a knee replacement a few years back. It's been great. Yes, I ride 50+ miles a day (seasonally). I've never done it 10 or 15 days in a row, but I have no doubt that I could. In the summer, I do 65-75 mile rides just about every weekend. I've done centuries. I've done long weekend rides (3 or 4 days) of similar mileage. I'm careful about gearing, and don't try to push huge gears (I use a Sugino 48-36-24 crank, so I basically don't have any huge gears), and I haven't had a moment's trouble with my knee.

Really, knee replacement works. Do it. You'll be happy. I went from a near-cripple to riding as far and as often as I want to. Even the surgery wasn't that bad. I went into the hospital on a Thursday morning, and was back at work the following Thursday.

Oh, yeah. Forgot. I'm 50 now, was 46 (or 45, can't remember) when I had the surgery. I was on the bike pretty soon afterwards. In fact, getting back in shape was my main motivation for getting back on a bike after too many years away.