I can attest to the fact that an improper seat height can cause some real problems. I had one of those 10 speed bikes with the thin frame and a small seat. I'd ride at least a ten mile loop around town every day and come back exhausted and sore. Other bikes didn't make me feel that way. I took my 10 speed to a local bike store and the manager asked me to mount it like usual.
He took out a measuring tape and measured the distance from my foot in the lowest position to the seat. He told me the problem was my seat was too short and my leg muscles were getting fatigued a lot sooner than usual. I wasn't getting a full stretch when I pedaled. He raised the seat a good inch or two, and I tried it again. It was twice as easy to pedal and I didn't come home sore. If you're having a lot of muscle pain after a bike ride, it might mean your seat height is wrong.