Just because one particular Lydiard runner (Halberg, in this case) was bested by other runners who used different methodologies doesn't invalidate Lydiard's training. We could just as easily turn that argument on its head and list interval-trained runners who COULDN'T best Peter Snell's 800m world record of 1:44.3 (on a grass track no less) and say that his training was superior. Heck we might as well just look at the training of the very best runners in the world today and say anything other than what they do is "trash." But, like you said, if we look at the training of the best in the world, what do we see? A large block of training which EMPHASIZES aerobic development (whether it's the winter training of Mo Farah or Galen Rupp or the "Fundamental" phase of Renato Canova) prior to training which EMPHASIZES race-specific development. Do I think that true-blue by-the-book 1962 Lydiard training is the only way to train? Of course not. Do I think any type of interval workout during the "base" phase is a mistake? Of course not. But does the legacy of Lydiard, which is indeed a focus on emphasizing aerobic development before race-specific training, inform the ideal training method (whatever that may be)? In my opinion, yes. I think Antonio and I (and Lydiard for that matter) just fundamentally disagree on the ideal way to go about training middle and long-distance runners. As far as what constitutes "hard," I take pains to avoid putting any numbers on that. I'm going to catch a lot of flak from Antonio and others for saying this, but I sincerely believe that a particular pace can be "hard" one day and not hard the next based on the state of mind, the body's state of recovery, weather, and so on. Lydiard would say that too much hard training is bad because it causes a constant state of low pH in the body. I personally think that too much hard training is bad simply because it reinforces an aggressive mindset and inefficient motions. To put it in another way, you can run the right pace but the wrong effort. Anyways, this is a bit ironic coming from me (given my formal training), but I particularly like this quote from Renato Canova on this topic:
Personally, I never met a scientist that was good coach, because they want to use athlete in function of training, not training in function of the athletes. Don't forget that the most important problem to solve is to make easy what is difficult, and for this goal we need to be very simple, natural in our approach, bringing our athletes to train more without too much pressure from hard workouts. That's the reason because too much hard training is a mistake, because athletics become a contunuous examination, no more a pleasure.