The only top Marathon without pacers is Boston, and are the athletes themselves deciding the speed of the race.
Without going again to speak about wind yes / wind not (I continue to think that in 2011 the wind was not continue, not directional and not so strong as many think, for justifying the, at that time, unbleiavable performances), the Boston times are mainly influenced by the action of some runner IN THE RACE (because, as I said, no official pacers).
So, in 2011 the race came from Ryan Hall (not from wind !) such as in 2014 for women came from Shalane Flanagan (and, in any case, the winner was with the leading Group from the beginning of the race).
When you speak about somebody winning coming from behind (Meb in Boston, and Wesley Korir), you speak about a race WITHOUT pacers, that is not built for fast times.
Another thing is to lose 20-40 meters for some attack of another runner after 32-34 km (for example, the even pace is 2'56" per km and a crazy Kenyan goes for 1 km in 2'45", you prefer to increase your speed from 2'56" till 2'50" but not to close the gap immediately, and after 3 km you, with your even fast pace, are able to overtake the leader, slowing down at 3' - 3'05" (and probably dropping out) because HE DID SOMETHING WRONG). In this case, the winner is not some athlete coming from behind, but some athlete Always in the leading Group, using his brain while somebody else uses his belly.
The fact that all the last WR were bettered with negative split, clearly explain why is not possible to win these type of marathons without staying in the leading Group from the beginning.
A little difference the situation in the marathons for women, with common start. In this case, we can see athletes running very fast from the start not winning the race, if some other can follow at her planned pace together with some male rabbit. This is what happened in Berlin : Shalane was Always in front WITH HER PACEMAKER, Tirfi and Feyse Boru followed some second behind WITH THEIR PACEMAKER, and at the end they overtook Shalane after 30 kms without staying together from the start, but in this case we can speak about athletes running AGAINST TIME only, following a precise speed, and the real race starts after 35 km only.