just to clarify, your seat post is at the right height when you have your leg extended but with a slight bend. not completely straight. i've been having a lot of pain in my knee and i was riding with my leg fully extended. as soon as i lowered my seat a little, the pain improved significantly. having it too high prevents you from keeping your hips stationary and also stretches out your knee (in a bad way) with every pedalstroke.

but as people have been saying, look into a brake or reconsider your gearing.