Yeah this is an interesting point. I definitely think that there are categories of runners who respond to training differently, like many have said. However, to start building pseudo-scientific categories (slow vs. fast twitch) in which to place runners in is to simple of a model that is hard to verify. There are far too many variables involved. As jngjerngerjg said the race speeds of some accomplished distance runners fit into what would be considered high level sprint speed for some high school athletes, and that clearly is a muddling factor. Especially considering that sprint ability is heavily neurological, and genetic (neural explosiveness does not respond well to training) it would appear that many of these 'slow twitch' athletes display really amazing fast twitch ability.
I don't think this means that categorizing runners is a hopeless endeavor. One of the positive things that makes the task of a coach easier than that of a scientist (I'm both, so I would think I have space to make this comparison) is that a successful coach does not necessarily need to be able to say what "exactly is going on here" but rather know just simply what works. Now of course science can help with that process, but typically the results drive the scientific inquiry. Take Lydiard for example, he had no idea what exactly was going on physiologically, but he trained some hella good athletes. Now we have a better idea why running easy miles is important for athletic development (a less damaging training stimulus is still a stimulus that drives certain adaptations, such as heart hypertrophy, capillary development, etc.), but also why high intensity training is absolutely necessary still (which focuses on the adaptations of blood buffers, muscle recruitment, psychological strength, etc.)
I think this whole conversation is attributing definitions to things we've observed as coaches, but we shouldn't take the leap from observations to scientific fact too lightly. Obviously, there are some distance runners who do better at lower or higher mileage, or taking easy runs faster or slower. The goal is figure out how each part of an athlete responds to the training, and continually adjust the stimulus to maximize the returns in performance and fitness. What this means is the stimulus must constantly then be increased to drive adaptation, which is why most elites have impressive volumes. More running = better runner, simply put. But the question of what that 'more running' means is an individualized question that a good coach has to figure out.
In summary, I believe the whole question this thread is trying to answer is the wrong question to be asking. The question is not whether "high or low mileage", but rather "how can I, as a coach, continually push my athletes to increase their training stimulus in a productive way to increase their race-specific performance?".