A bicycle is a form of assist: mechanical assist. Try this first experiment. Try running down a hill. Measure power output vs speed. Then, ride a regular bicycle down the same hill. Measure power output vs speed. Which do you think is faster? Duh.

Now, try a second experiment with a standard bike. Measure your watts vs speed while walking or running and then measure power output vs speed on a regular bicycle. You'll be much, much faster on a bicycle with the same power output.

It is extremely easy to achieve 12 mph on a bike shop bike on flat terrain. It is extremely difficult to sustain a speed of 12 mph on foot for any distance.

A regular bicycle is disturbingly efficient compared to walking or running. A regular bicycle is a form of assist. Therefore, using a regular bicycle is a form of "cheating" compared to walking or running.