Most athletes work in short burst and long duration cardio is counter productive for most athletes to optimize their metabolic energy systems.
Charlie Francis states in: Training for speed “Enough power related work must be done during the early years, ages 13-17 to maintain the genetically determined levels of white or power related muscle fiber and promote the shift of transitional or intermediate fiber to white, power related muscle fiber. Endurance work must be carefully limited to light or light to medium volumes to prevent conversion of transitional or intermediate muscle fiber to red endurance muscle fibers”
Lesson: Making young athletes perform low intensity cardio for long duration is not optimal. Short high intensity efforts will allow them to improve “fast twitch” fibers and also helps improve VO2 max, which will allow them to reach optimal performance. Get to that 85%+ intensity