Going off of 2019 Marathon. Just before dawn the air was so thick it felt like a damp blanket. I was a G start and on the road to MK, so that would have been just after 6 am. As the sun rose I could see the moisture hanging in the air, and was fogging my glasses, that would have been the TTC parking lot, mile 3 at about 6:45/7 am. Started struggling with the heat in mile 16, so best guess about 10 am and by the time I was handed the sponge in WWOS (mile 20) sometime after 11 you could see people starting to drop and the aid tents were full.