Less than one-tenth of one percent of the general population is capable of attaining such a feat.
That's a pretty powerful statement, and Fatty uses it to assert that only the upper level of pro athetes can overtrain (it's not clear to me if his statement is another Friel quote, or a paraphrase, hyperbole, or the product of too much of "the best cake in the world")
To me, what it comes down to is this question:
Is my overall fitness level going to be better if I:
a) train today?
b) rest today?
If the answer is "b" and you train today *anyway*, you are overtraining. Of course, there are some caveats - your "training" today might be more "active rest" than training.
So, how many cyclists overtrain? Well, my experience is that many people - especially those who like to push "until I start getting tunnel vision" - tend to have trouble controlling their intensity. Friel says:
Generally, a week should have at least as many recovery workouts as hard workouts, if not more. Every third or fourth week there needs to be a period of greatly reduced training with an emphasis on rejuvenation.
Carmichael says something similar.
So, anyway, my point - and there is a point this time - is that many - if not most - serious recreational athletes are in danger of overtraining now and then. I have a friend (no, really, a friend...) who would ride his bike "all out" for 75 minutes every night for a period of months. I don't see how he could be anything but overtrained.