Is it time to rethink the conventional wisdom that 70-80% of endurance training should be spent in Zone 2, and instead adopt a more polarized approach, where athletes dedicate a larger proportion of their training to Zone 5 and above, with a corresponding decrease in Zone 2 work, in order to better reflect the demands of modern racing and the benefits of high-intensity interval training (HIIT), or would this approach lead to a significant increase in the risk of overreaching and overtraining, particularly in age-group athletes and those with limited training experience, and are there any studies or data that support the idea that a more aggressive approach to training can lead to improved performance gains without a corresponding increase in the risk of injury or burnout?