LD - some good points raised there. Bear with me, this could get long
You have misread some things though. I didn't say anything about 10 years of hard training, I said that he believes it takes 10 years to build your aerobic house before you are ready for hard training. The simple fact is that the Kenyans, much like the British/Irish runners of yesteryear have a much better developed natural base than we do.
I guess, overall, my main point is this - how do you improve? As I see it, you provide a stimulus to the body, the body has to adapt. Once the adaptation has taken place, re-applying the same stimulus in the same manner will simply produce the same results, so the stimulus has to change to bring about further improvement, otherwise you will plateau.
In Canova's case, his runners' progression tends to eventually see them running the marathon after serving an apprenticeship at shorter distance (similar to the idea that you run 800m when you're young and marathons when you're older, but with a quicker progression). Within that approach, the marathon training will change as you become a more mature marathoner.
As an aside, I only gave Canova as an example of how training can evolve. I didn't say it was radical, or the 'right' way. How it differs from other training though tends to be that it funnels paces towards specificity as you approach your target race.
I think you'll find that Bill Adcocks ended up retiring due to knee problems related to his job, so there is a lot to be said for a more relaxed working environment (e.g. office work as you've pointed out). Bill Adcocks training also evolved over time, incidentally, thereby providing a constantly changing stimulus.
In respect of runners aged 19 or 20 who have run a sub-60 marathon, well I've had a look, there are 14. Of those, at least two (Eric Ndiema and Geoffrey Kipsang) had serious question marks raised about their ages. One of them achieved the mark on an assisted course, and only 3 others were actually under 20 (Wanjiru being one). None of them are names I recognise as being trained by Canova.
Running 3hrs a day? Yes, Ron Hill and Dave Bedford might not be the best of examples, but as you've said, there are always outliers - Geoffrey Mutai's standard training As I understand it, this is what he has progressed to as his standard week. What he done, training wise, to get to this stage I don't know, but it is interesting as a stand-alone moment in time.
Like Njord - I would be interested to see your standard training. I've identified some deficiencies in mine, and some of what I was previously doing sounds like it could be similar to yours. I beginning to think more and more, that the key to maximising your marathon potential is to improve your 10k potential. I've found a surprising amount of commonalities across various approaches that seems to back it up, so that's personally where I'm heading as I, similar to you, don't run good 10k times relative to my marathon time (my mara/10k conversion comes out as 4.5, which is just incredibly tight).