Originally Posted by
Nickel
I can understand that only doing z3 would be detrimental to gains, so I was wondering if that was what the book is referring to?
Probably referring to the habit of many to go "hard" all the time, because if one does that, one never goes hard enough, always being too tired. In a huge and probably foolish generalization, one should go either easy or very hard. For training, that is. In a long event, one will spend a lot of time in z3. But you'll get faster in z3 not by spending a lot of time there, but by riding in zones 4 & 5 and then recovering in 1 & 2.