Nobby and HRE are right.
BTW : The first time I came across this 20% deal was here on letsrun. I wondered what and where on earth that came from !!. I had never heard Arthur talk in percentages.
As for my take on building mileage. What I did is gradually build my Sunday runs up so I spent more and more time on my feet. That did not mean I neglected the rest of the week.
I had this theory that if I had a "solid" Sunday behind me then the rest of the week 'fell' into place.
I also learned from Arthur (And many of his "Boys") that once you have run 20 miles you can always do it again. No matter when. I now sincerely believe that.
Just this Sunday I headed off on my own for a run though our forest and found that the further I went the better I felt and one of the thoughts I had during that run was about this exact aspect. I only went for an Hour and a half but if someone had come along and said "Lets cruise for another hour" I could have easily done so.
The body has never forgotten. It just needed to be taken to that "spot" where it could do it again.
As for the rest : the key is consistency of running days per week/weeks per month/months per year, then years upon years.
As I said before on another thread when asked what the 'fastest' long run I ever did was. It was 2:40 marathon pace for 22 miles. It felt easy. BUT it took 4 years of cocnsistent running to get to be able to run that fast that easily.
Hope this helps