The legs being wrecked more is true in my experience, but doesn't necessarily mean it's a better training load. It's taken me a while to get my head around that. This comes down to what sirpoc was describing at the start, the hard impact and feeling of the body the real speed workouts bring, but in the bigger picture not really leading to much improvement or possibly stopping you hitting the sessions that do make the difference- note his stagnation of a reasonably decent amount of time and the boom and bust cycle others have mentioned here, before he started training like he cycled.
I'm coming well around to the idea that whilst the 5k might feel harder , the 10*3 mins workout is actually banking more overall benefit AND you remain fresher. I guess here for runners around 18-19 mins or under. I think if we are talking guys slower than that, it's a different story.
Also, because I bug sirpoc all the time on Strava I followed up the quote above about how 'easy' he means. The answer I got was easy means easy and is easy, in COMPARISON to other running systems daily/easy/long run days , or what might get called steady runs etc. So it was much easier than what he had been prescribed by running plans before, I wholeheartedly agree with this. In this sense he seemed to mention 80/20 or Daniels that he said were remarkably hard in comparison and would quite quickly lead to problems if he followed the medium to high end of those "easy" day paces or efforts.
At the end of the day there is obviously a huge amount in this and it clearly works for a vast number of people now, just look at the testimonials not just here but all over the internet. I would still ask why from a scientific point of view and also how remarkable it is the balance in general for most people for the original sirpoc lay out of 3 ST-3 EASY- 1 LONG seems to work.
I don't know exactly but the Strava group, here, Reddit, Facebook group I have seen dedicated to it, you have to be looking at 70-80% of people sticking to it improve over a decent amount of time as a conservative estimate. On top of that, some of the jumps from even seasoned or long time runners are almost remarkable. The bit take I can see is we are moving away from the ridiculous boom and bust cycles and allowing people to balance on the cautious side of where freshness lives which then seems to be allowing people to increase load beyond any other plan has ever allowed them before, whilst maintaining the 3 workouts which seeminglly whilst easier, aggregate in more than the equivalent of a 2Q week.
I will add I am incredibly fast twitch, I saw that debate - but I also have absolutely smashed through walls and boundaries in my second life as a runner. I've run 16:33 which is almost my best from college and I'm 37 with a family of 3 to look after and all the stresses that brings. I've totally ditched absolutely everything but the 3 sub Threshold sessions a week and the easy. I was pretty happy with my turkey trot 18:21 in my 30s but absolutely delighted with my 16:33 a year and a bit on. Shout-out again though to sirpoc himself, always happy to help and answer even though I'm sure he's fed up on Strava and pushed me onto the right path when I had some questions. TL;DR of this bit, whilst I maybe took me a few extra weeks to adapt than others have suggest. This is absolutely for older fast twitch hobby joggers like me as well.
Oh and good luck to sirpoc in the London Marathon . I'm also feeling good that at the ripe age of 41 he will break 2:30 at his first attempt. As long as he doesn't do anything stupid , 2:30 is almost a worst case scenario. I also don't know how many non-former elites break 15 in their 40s, but I think he will soon join that very small fraction of a % of runners.
Really enjoy the thread, thanks to all the other top posters as well. From shirtboy, unbelief keeping everyone on their toes, Coggan when he's in good spirits and not wasting time with lexel, big shout-out to Hard2find as well, some amazing resources. I'm sure others should be included, but they are the people I think of.