I'd go along with what Leob said. "My completely uneducated guess is that the muscles that support the knee joint stiffened up from the way you where standing, and it's a one time thing."

Lay off it a few days and if you can walk fine, etc. get back on a take it easy. Try easy flat 10 milers and work up from there. At least if you do this you can't possiblty do yourself any harm.