Regarding the knee, I agree that you should use a higher cadence, but also check bike fit. My knee hurts if I ride a bike with a too low saddle.
To quick-check saddle height: ride with your heels on the pedals. Each leg should then be straight when in the lowest (six o'clock) position. When you change to a normal riding position (balls of feet on the pedals), the legs will be bent slightly when in the six o'clock position.
When saddle height is correct, your feet will not reach the ground when you stop the bike. Also, you will have to "climb" into the saddle when you mount up. On a correctly fitted bike, starting and stopping both take a little getting used to.