I ride with tennies with no problems, but have recently begun to experience knee pain after changing to a new saddle. The height is fine, but I'm going to continue to adjust the front/back position, this could be your problem. Not being correctly over the pedal can cause knee pain. Ask LBS for help if need be.

Also the cramping could just be dehydration and not neccessarily a potassium shortage. Keep general hydration up with plenty of H2O on rides. Best source of K+ is orange juice (banana's don't really have that much). Also a good source of K+ post-ride is a baked potato (eating skin is best).

Cindy