Daniel's analogy of the typist is perfect. There are guys who run better off 100 mi base weeks and others who run better off some variation of 5 pace theory.
One of the smartest things Frank Horwill ever wrote was to not simply repeat the same training over and over if it is not working. In other words, if you and your athletes seem to hit a wall after running the 100mi weeks, then next season try the 70 mi weeks at much higher tempo. And of course, go the other way if you are not making satisfactory progress with 70 mi weeks of fast running.
I posted something last week that didn't get the response I hoped, but perhaps all who are interested in this post might comment.
I referred to the chart of Herb Elliot's training in Lore of Running at age 18-19. Essentially, it was 4 hard days plus 2 days of sprint work and one day of rest. However, one of the hard days was really what we now refer to as the 20 min tempo run. The other interesting day is 10 mi hard.The total was only 50 mi per week.
In my opinion, this is not too far from what Webb is doing and is particularly suited to 1500m runners with natural talent.
I coached a high talented hs kid many years ago on a traditional model. He did run 3:50, 1:51. The one thing we didn't do was hard tempo in the early to mid point of the season, and I think now that if I had included a 5k run in 16 minutes every 2nd week he might have run 3:46.