I haven't read this whole thread so someone might have covered this already, but I get the feeling that a lot of the claims made by successful runners about succeeding on X amount of mileage are oversimplified or often cases of revisionist thinking rather than something closely resembling the intensity of work actually done.
The example of Mark Nenow came up recently, and his training method has been boiled down to 'run 13 times a week, 10 am, 10pm, 20 Sunday'. If a 33 min. guy reads this and thinks...damn...if I do that I'll run 27:20, he's sorely mistaken. What we're not seeing is the buildup, the racing schedules (read speedwork), the intensities of those 'steady runs' (in Nenow's case I think a lot of that running on hills at sub-5 pace counts as interval work same as if they were run on a track) and a myriad of other training elements which mask the true complexity of an apparently 'simple' formula.
We often hear about people running nothing but mileage for 3 months and running a PB, but what we often don't hear about is the work leading up to that. I ran a pretty good 10K one fall off nothing but basework for 2 months. I figured that basework might have been better for my running until my coach pointed out the intensity of the track work that I'd done in July and August. To conclude that mere mileage was my key to success would have been wrong.
I really believe that 99% of the people who claim to find no success in a structured program like Daniels', where reps and intervals are heavily utilised, are probably not really doing it correctly. I used to see it in university all the time - people running their intervals too fast, or not doing enough miles, or time-trialing the long run route etc.
There's a reason why runners move up in race distances over time. It's because the speed and fitness gained from the middle distances help in the longer distances. Is it any wonder that Dieter Baumann, Ali Said-Sief and the like have 3:30 1500 speed to their credit?
If all these high-mileage-as-the-road-to-fitness people are right, why aren't marathoners winning 10ks? It's always the other way around...