Great question. I have had many friends who have had the problem of running ridiculously fast. They would go on claiming how fast they ran their 100 mile weeks, pushing sub 6 pace almost every run. It worked in the beginning, and they would be workout warriors. They would run well in the beginning of their season, then quickly die out. By the time regionals came around they were done.
Yet, recently one of those friends finally slowed down. He was tired of the countless days running so fast he couldn't function in workouts. His sub 6 minute days, slowed down to hover around 7 min pace, and even slower on doubles. He said he never felt fresher. He kept his mile up all through season, ran his workouts just as fast as he did before, but he felt great all the time. He had the season of his life, and is only to seem to be feeling better and better.
Now, as he did have improvements in his "fast" days, they died quickly. Yet, he would keep up with the "fast" days. So after a few years of these days, he finally slowed down, and has seen more improvements and feels fresher. This seems to be more long term.
So maybe a more complex version of the question is, when he slowed down after his "fast" years, was he only maintaining his fitness from then? Would he be just as fast now, if he had never had those few years of what seemed like detrimental running?