The average cyclist racing on flat terrain can bike 1 mile in about 3.5 minutes, and it would take about twice as long to do so on mountainous terrain. In addition, the average commuter can bike 1 mile in about 6 minutes. The average professional cyclist is much faster.
A professional cyclist can bike 1 mile in around 2 to 2.5 minutes on flat terrain and in around2.5 to 3 minutes on mountainous terrain. The reduction in speed on mountainous terrain compared flat terrain is much steeper for the average cyclist than for the average professional cyclist due to skill level.