Saddle height?

“Lean against the wall, sit on the saddle, then hang both feet straight down. If your saddle height is correct, your heel should just graze the bottom of the pedal stoke.” –Todd Carver, Retul, a bike fitting company.
If you have pain in the front of the knees after the first few rides, your saddle is probably too low.
If you feel pain in the back of the knee, drop the saddle a little.