Training at this intensity maximally overloads the slow-twitch muscle fibers, increasing endurance. Since these fibers produce most of the energy, and create most of the power for any endurance event lasting four minutes or longer, workouts at this intensity should comprise most of your training. Note that training in this zone is the most effective way to overload the endurance fibers, and that training above this intensity is less beneficial for this purpose.
At low intensities, fat is the primary fuel for exercise. Obviously, this is important for body-fat reduction but it is also important when training for events of two hours or longer. The body stores one to two thousand calories as carbohydrates, but even the leanest athlete stores many thousands of calories as fat. Carbohydrates will always run out before fat. Therefore, fat is the ideal fuel for long distance racing. Training at this intensity increases fat-burning and decreases carbohydrate-burning.
Athletes training for shorter events with higher intensity need to perform much of their training at this level to stimulate improvements in the slow-twitch fibers. Performing basic aerobic workouts at too high an intensity reduces the effectiveness of harder workouts on subsequent days by fatiguing and/or depleting carbohydrate stores of the fast-twitch fibers. Training in this zone will feel very easy. For many people, the most difficult part of following a systematic training program is keeping the intensity low enough on easy days and long workouts. Staying in Zone 2, when appropriate, is critical for everyone. Going too hard on easy days is the number one cause of overtraining.
In terms of numbers, Zone 2 measured with a power meter is between 60-75% of your FTP. It’s “all day” pace, or classic long slow distance (LSD) training. The sensation of leg effort/fatigue is generally low, but may rise periodically to higher levels (e.g., when climbing). Concentration is generally required to maintain effort only at highest end of the range and/or during longer training sessions. Breathing is more regular than at level 1, but continuous conversation is still possible. Frequent (daily) training sessions of moderate duration (e.g., two hours) at level 2 is possible (provided dietary carbohydrate intake is adequate), but complete recovery from very long workouts may take more than 24 hours.