There was more fast running in van Aaken's system than many understood, partly, I believe because van Aaken presented what he did in a way that emphasized slow running. For example, he described a workout of Norpoth's that consisted of 10x400 in 2 minutes each, followed by 2km at a minute slower than his best time, followed by another 10x400, another 2km until Norptoth had run 15-17kms.
It sounds slow, but the 400s were really 350s done with a 50 meter walk. That means the 350s were done in one minute. That works out to 68.5 per 400. And Norpoth best 2,000 was around 5 minutes, so he'd do about 6:00 in training. So Norpoth was doing roughly 10 miles in that sessions and more than 90% of the running was between 4:34 and 4:50 per mile, not "fast" for a runner of Norpoth's abilities, but very good paced running for a reasonably long distance.
Manfred Steffney was one of van Aaken's runners and wrote a book on marathon preparation using van Aaken's principles and the training is not all slow jogging. There is a respectable amount of faster running, but always with an emphasis on controlling the tempo on the faster runs, keeping the effort below all-out, and always surrounded by plenty of easy distance runs.