FWIW, DH had to ditch his Speedplay Frogs because they hurt his knee. Even with plenty of float, your knee's getting torqued when the cleat is pointing a different direction from the way your knee wants to track.
A three-bolt design like Look pedals allows you to adjust the angle of the cleat. Trial and error can get you pretty dang close on cleat alignment if you're not ready to spring for a proper fitting.
And... time may prove me a liar... but I don't believe that knee pain is inevitable with advancing age.
Speed comes from what you put behind you. - Judi Ketteler