It's about endurance, not strength. It is a totally different game. If you go too fast, you'll burn up your carbohydrate stores and your muscles will develop crippling fatigue. If you keep your power output below your aerobic threshold, muscle fatigue will not be a problem for a long time. I had my aerobic and anaerobic thresholds and my VO2 max measured at a test lab back in 2001. Don't remember the figures any longer. They correspond to a certain heart rate for each individual. I think my aerobic threshold was 120 beats per minute. I remember keeping it between about 110 and 120 during most of that trip, except for the final 40 km when I wanted to test myself.