Joining Marathon soon

My knee is so much better I'm thinking of joining in the upcomming marathon!   Yeah, right.  Seriously tho, it continues to improve each day.  I actually got out of the house yesterday to play cards with ladies who are also falling apart.  They don't mind if you take a break between games and fall out on a sofa.    My plan is to try to get doc to do the other knee.  He is determined to do replacement but I am reluctant to what I think of as the amputation of limb.  I am also incredibily lazy when it comes to excersize.  If he could possible just make improvement without it, I would be all for it. 
Anyone out there have a knee replaced?  My info source here says there are different knees.  Some off the shelf and some designed from MRI.  Never having considered it, I haven't done any research.  I am still amazed that I walked around with torn meniscus and ACL for years without one of these brilliant overpaid docs diagnosing..I guess if you are still moving and not screaming they think arthritis and fat.  Anyway, that's all in the past.  I go Monday to get stitches out and beg for MRI of other knee.  Wish me luck.