BOSTON — They started before dawn, lining up in the fog in Hopkinton like they do every Patriot's Day. Thirty thousand of them. Some built for this. Most not. All of them, apparently, voluntarily choosing to run 26.2 miles. In April. In New England.

Welcome to the 130th Boston Marathon, the oldest annual marathon in the world and the only road race on earth that makes elite runners qualify just to enter. If you think that sounds grueling, you're catching on.

This year's field includes defending champions hoping to repeat, a fresh crop of Kenyan and Ethiopian distance specialists who make the rest of us feel bad about ourselves, and the usual contingent of charity runners who have raised millions for causes ranging from cancer research to local food banks. The wheelchair divisions set off first, as tradition demands.

Crowds lining the route — from Hopkinton through Framingham, Natick, Wellesley, Newton, and into Boston — are expected to top 500,000 spectators. The Wellesley College Scream Tunnel will do what it always does: greet passing runners with the kind of noise that could shatter glass and actually, briefly, make those Newton hills feel possible.

Weather this year is cooperative by Boston standards — cool temperatures and overcast skies, the kind of conditions runners actually want. No one wants to run 26 miles in the sun.

The elite men's and women's fields are stacked. Kenyan runners hold a commanding historical edge in recent years, though organizers have noted growing depth from European athletes who have been quietly closing the gap.

Results are expected throughout the afternoon. The real ones — the back-of-the-packers, the charity runners, the people who signed up a year ago and have been regretting it ever since — will finish long after the cameras have gone home. They always do.