"A sub 2:06:00 time for a marathon (male), may not be possible without the use of performance-enhancing drugs. I recall, reading an article on the internet, which I can no longer source, in which a high-level European male marathoner, with a personal best time in the 2:07's, was under no illusion, although he couldn't prove it, that this is the case. There is no one better qualified, to provide opinions, expressed as facts, than those athletes who are participating at the highest level."
I would venture to say that even a sub 2:10 for a marathon is already in the grey area. The first person to run a sub 2:10 was Derek Clayton of Australia, all the way back in December 1967. He broke the record by almost 3 minutes, which was set just two years prior.
A 2:01:39 (official) marathon is ridiculous. The last 5-10 years has seen a lot of top Kenyan runners and coaches busted for doping. While Kipchoge is very talented, I don't see him running clean for that WR.