But as the time or the pace of the activity increases, your muscles begin to rely much more on glycogen and much less on fat for fuel. Unfortunately, your body's glycogen stores are in short supply compared to fat. So, after about an hour or two of exercise, your readily available glycogen supplies abruptly run out. You can go no further. Consuming coffee before the exercise helps here.
Caffeine stimulates an increased use of fat as fuel during the exercise.
This decreases your reliance on glycogen. And you can cycle longer and harder before exhaustion. The actual effect of caffeine on real life, open road cycling is not known. The caffeine effect depends on how much you habitually consume, or your tolerance for the drug. People who regularly drink two or more cups of caffeinated coffee a day may have to abstain from the stimulus for four or five days for the caffeine to help improve their performance.