The key to running well on Monday was to be well hydrated during the morning (maybe even over-hydrated) and to drink early and often.
Most people who I train with who went out looking for a time in the 230s and 240s will take fluids every other mile or even every 3. You had to go every mile. Didn't need to be a large amount of fluids, but you needed something.
The heat in the first 8-9 miles was direct and noticeable. In Wellesley and Newton there at least was a breeze to help cool off, but for most by then, it was too late and dehydration was already setting in.
Around here in Boston, it was a very tough day for local runners to try to race. We had maybe 10 days where that kind of sunlight had been present and felt strong. The problem was that we were stuck in the 40s for the 6 weeks leading into the marathon. There was one Sunday when we got up to 60 degrees in March, but that was it. The weekend before the marathon, I remember it was in the 30s. Just such a noticeable difference.
I will say that I've read comments on these boards from people claiming they had no knowledge these conditions were coming and were surprised that morning that it ended up that way. I can remember this happening in 2014 - we were expecting a high of the mid to upper 50s and temps hit 70. But this year, the news reports had reported for days leading up to the marathon that it was going to be warm until we hit Boston, where the seabreeze would overtake the temps and conditions that were more inland.