Entering the sub 2-hour marathon debate

Background

The two-hour marathon: Can it happen? The debate heated up with Dennis Kimetto's time of 2:02:57 in the fall of 2014. As I recall, he was less than a kilometer from the finish when he crossed the two-hour mark. But even before Kimetto's run, Alex Hutchinson had already assembled a discussion on what it would take to run a sub 2-hour marathon.

I have been watching these debates mainly from the sidelines as I hadn't found the data convincing enough either way. The only truly convincing (though most difficult) demonstration would be to run a 1:59:59 marathon. The easiest -and most problematic- line of reasoning is to plot marathon record time vs date achieved and extrapolate to one's peril:

Image from SweatScience's post
2032: Year of the Sub-2:00 Marathon?

This approach is much less unappealing, introducing no additional understanding of physiology or innate performance ability. Such extrapolations would never have predicted advances in the high jump, swimming, or speed skating. The curve itself is also a questionable line-of-best-fit.

Qualifying times

A 2009 New york times article asked whether slow runners were spoiling the prestige of the marathon.

Many of those slower runners, claiming that late is better than never, receive a finisher’s medal just like every other participant. Having traversed the same route as the fleeter-footed runners — perhaps in twice the amount of time — they get to call themselves marathoners.
And it’s driving some hard-core runners crazy.

If you are one of those going crazy, what to do? In order to surround yourself with fast(er) people, there is at least one option: the Boston Marathon. With its aged-based minimum entry standards, there is a certain density of speed not found in many races. Because Boston marathons have to already have run a marathon, a huge number of feeder races contribute to would-be pilgrims looking to qualify. It's a reasonably symbiotic relationship since many races can advertise their BQ potential. Yet it still leaves Boston's entry policy as popular yet unique. What other races do likewise? I googled "What marathons do you have to qualify for?". Here's what I found: