Costes is a great example. I suppose technically he was the first American under 2:20 but that was on the old Boston course that was nearly a half mile short. Another example from a little later was Lou Castagnola who was the fourth American under 2:20 on what was a full course. He told me he ran 6 days a week, nearly all of it intervals on the track totaling 50-60 mpw. Norm Higgins, who was the second (I think) American under 2:20 on a full course, would be another example. I'd bet if you go back to the early 60s and before you'd find a few more examples of people running marathons on nearly nothing but intervals.