I am fairly new at cycling and there is something that I don't understand about the training aspect of it. I have a long sports history (I was a professional swimmer for most of my life). Now that I got into cycling I keep reading that zone 2 training is the preferable method of training. I can sustain zone 4 training for 3 hours of a ride and I don't understand why to train in zone 2 when I can train in zone 4. Wouldn't it improve my endurance and vo2 max if I train with higher effort?