The most common mistake made in cycling and training for cycling, especially among casual cyclists, is to pedal too slowly while in a high gear. Instead a lower gear should be used and the pedalling rate increased.
Cadence is the term used to describe the rate of spinning in rpm. This the key to fitness and specifically to increasing aerobic fitness. To used an analogy, in body building low repetitions and heavy weights (anaerobic fitness) help build muscle bulk however do not improve stamina. Lower weighter and more repetition (aerobic fitness) increase stamina and general fitness.
Effect on muscle and blood pressure.
When riding with a good cadence (ie. more revolutions and with less pedalling pressure) and you contract the muscles, the muscle cells shorten and get fatter. When they get fatter, they put pressure on surrounding blood vessels forcing the blood out of them and towards the heart in a forward direction. This accelerates the flow of blood through the body while decreasing the workload on the heart, if the contractions are short and fast.
When riding in a higher gear with less pedalling revolutions the contractions are significantly longer. This causes a backpressure against the heart forcing the heart to work harder and slowing the blood flow through the body. This backpressure is OK for the short distances a racer may need to make a break, jump a gap or chase a group but it's work against the body for longer distances.
Speed and bicycle control
It is easier to control your speed and keep it smooth and steady when you spin. Pushing a high gear reduces the feeling for speed and your speed subsequently fluctuates up and down which results in quick loss of energy. Spinning will also improve your bike control.
It is generally thought that a cadence, the rate of spinning, should be between 80-100 rpm.