Runners boarded buses early in the morning at Boston Common, while spectators had already arrived in Hopkinton (below) by 7 a.m. (Top photo by Steve Silva; below by Zuri Berry)

"The one thing that we can't control is obviously the weather," said McGillivray, who appeared on Boston.com's live video broadcast from Hopkinton this morning. "The runners themselves need to heed all the cautions and the warnings that have been sent out there. ... Maybe it's not a day to set a personal record."

With record heat in the forecast, Boston Marathon officials have been very clear in emphasizing to runners that today should not be treated as a race, but rather an experience, and if the experience proves to be too much, runners need to make the smart decision.

"What we're doing ... is increasing the fluids out on the course itself, almost doubling them, more ice on the course at the water stations and the Red Cross stations, even in the emergency rooms at the hospitals along the course," McGillivray said.

He added message boards would be stationed along the course to encourage runners to go slow, and fire departments would set up sprays to cool runners.

"If unfortunately they do get in a little trouble, we have a medical committee and a medical staff second to none to take care of them," McGillivray said.

We'll have coverage thoughout the day here in the Marathon Blog, and live video is planned from Hopkinton at 7:15, 8 and 9 a.m.

Stay tuned. The marathoning begins at 9 a.m. with the mobility impaired program start. The elite women go at 9:32 a.m. and the men at 10. The final wave goes at 10:40 a.m.