One hour of bicycle riding can require 500-900 kcal of energy, depending on the speed, the terrain, and the weight of the racer. An individual wants to lose weight by riding at a modest effort, consuming 505 kcal of energy per hour. How many hours must he ride to lose 21.0 pounds of body fat? One gram of body fat is equivalent to 7.70 kcal of energy. There are 454 g in 1 lb.