I ride fixed, over 50-100mile distances often. It's a workout. I wouldn't say more so than a geared bike, as a fixed gear and geared bike allow for different physical challenges...it's just different. I know a few roadies that race that train on Fixies or SS in the winter. One thing it will teach you, especially a fixed gear, if how to spin and a proper pedal stroke (along with keeping your hips steady on the saddle).