The three-stage event will include a swim from Alcatraz Island to Marina Green, a 25-mile bike ride through the hills of San Francisco and a 7-mile run that includes a 400-step sand ladder.
Ben Collins, of Seattle, Wash., won the men's course and Tenille Hoogland, of Ottawa, Ontario, won the women's course in 2010.
The event begins at 7 a.m.