This is a timely thread for me as I just recently started experimenting with this kind of training after a string of very disappointing races where I didn't perform nearly to the level that I felt I was capable of. I'm 31, and all of my PRs are from college 10 years ago, but I felt I was near the level of fitness I had in college before jumping in to this training, so I'm interested to see if I can cross off any of my old PRs.
I think it's super, super important to keep the intervals at the prescribed paces, especially the 1k and 2k reps. Klaas actually does say that HM pace is okay for the 1k's, but I've felt more comfortable doing them somewhere between HM and M pace. Still super early, but my legs have already been feeling a lot better with a lot more spring/bounce, and I've noticed that my recovery jogs in between reps have gotten faster without any additional effort.
Overall I'd say it seems pretty promising so far. I plan on sticking with it for at least a few months. The only thing I've changed from the general framework is that instead of his weekly "mixed session" I plan on doing a weekly track workout with my club on Saturdays (or a race depending on the week). This is important for the social aspect, and I think it acts as the one "hard" session during the week that is recommended in the book.