I have to disagree with this. I used to use Look pedals which offered a ton of float. I still had knee pain at the ends of long rides. I'm now using SPD pedals which offer almost no float and I have no pain. It's my belief that pedals with too much float allowed my knees to track through less-than-ideal positions during the pedal stroke. Repeat that poor pedal stroke 6000 times/hr for several hours in a row and you end up with knee pain. SPDs, on the other hand, force my knees into a more ideal pedal stroke... and keep them there. Not saying that clipless pedals will cure all knee problems, but they're worth a try.

It definitely helps if you can find a good bicycle fitter to work with. If your fitter is good, it also helps if you give their suggestions a try. I found that ideal better footbeds along with appropriate shims made a noticeable improvement in my comfort on long duration (>4-hour) rides.