Of course it's not too late. But if you need the replacement, get it--NO WOE can correct bone-on-bone.
The fact is, though, that you HAVE to lose weight or the knee replacement won't work. It's the extra weight that tore the knee up--as a nurse you KNOW that extra weight = extra wear and tear on joints. If you get on this plan and lose the weight--try to get to a LOW normal BMI if possible b/c that will put the least strain on your joints--you will stand a much better chance of gaining as much benefit from the knee replacement as possible, having it "work" for the longest possible time, and avoid having to replace the other one, too.
I know someone who got both knees replaced but never lost the 100+ pounds she needed to lose. In fact, I think she's gained some more. And she is no better off than she was with her old knees. Not surprising. Until you get that extra weight off your joints things just aren't gonna work right.
Don't forget, as well, that this WOE will protect your heart, prevent you from getting diabetes, etc. etc. etc.
So just DO IT NOW. No excuses! We're here for you!