Just a tip, you aging exercisers: knee pain is sometimes caused by a lifetime of subtle foot/gait issues. Be sure that you have your feet and gait checked out before deciding on knee surgery. Not all orthopedic surgeons are attuned to this issue, and I’m not a big fan of podiatry.
If you have a full service athletic shoe store, that may be your best bet. They can generally see any significant gait problems. Be sure to take along a pair of old shoes so they can see the pattern of wear.
They may be able to suggest a shoe or insert that will normalize your gait and thus take the pressure off the part of the your knee that is wearing out.
ps: surgery is a very serious step and shouldn’t be undertaken without a second opinion; knees are bad enough, but don’t let anybody operate on your feet.