"So, the way that slow running makes you fast is because if you’re doing a
moderate pace – and by moderate pace I’m – my experience is 7 minutes a mile or faster,
NOT SLOW PLODDING (my emphasis). You have to be moderate, at least. And you run for a sufficient
duration, where if you run long enough, eventually the fibers that are being recruited to
handle that pace will become depleted of their glycogen stores, and they can’t continue to
contract"
This is on the bottom of page 3 on the PDF that Hodgie-san linked to. He is specifically defining 7 minute pace as "moderate," not plodding. (By the way, Hodgie, thanks for putting that up. I actually lost my original when the computer it was on died.)
On the next page there is this in which he draws a clear distinction between what happens at 7 minute pace and what happens if you go much slower than that:
". Now, if you’re doing it like these very long distance runners who run
somewhere between 8 and 9 minutes a mile pace forever, then they probably never get to
those fibers because they’re able to do their exercise utilizing fat as a fuel to a large
extent. When you exercise at a higher level, i.e., 7 minutes a mile and faster, then you just
can’t do that on fat. Glycogen has to be used, and eventually it becomes depleted in the
fibers that are being used. And when that happens, others get called in to take over"
If you read a bit further there is a passage where he talks about starting off his base period doing long runs at 7:00-7:30 pace, going faster on the shorter runs and having the long run pace come down to something in the neighborhood of 6 minutes as the base phase moved along. At no point does he ever mention that they were "plodding" or that he would change that.