The fitness component most improved by interval training is cardiovascular endurance. This training method involves alternating between high-intensity bursts and low-intensity recovery periods, which effectively enhances the heart's ability to pump blood and improve oxygen delivery to muscles. Interval training increases aerobic and anaerobic capacity, making it a time-efficient way to improve overall fitness. Regularly incorporating intervals can lead to better performance in both endurance and high-intensity sports, while also promoting fat loss.