Remember how free you felt on your first set of wheels? Riding a bike is just plain fun, and it is not just for kids. Riding for fitness means you ride with intensity, specifically with speed and for time, or using hills to build strength. When you ride with intensity, bicycling is an aerobic activity that uses your hip, buttock, and thigh muscles. There is no pounding (as there is in running), and as long as you have a proper fit with your bike, you may feel as though you can ride forever.