Jonathan Mederos set a personal record in Disney's Princess Half Marathon last year, winning first place with a time of 1:16:17.

The reception he got was far from his personal best, though. "When I crossed the finish line, the announcers were silent, and I got downcast looks from the crowd," says Mr. Mederos, a 25-year-old high-school philosophy teacher in Miami.

That's what a guy gets for winning a women's race. As Rick Cordes took last year's Nike Women's Half Marathon, he slunk across the finish line, with a finger pressed to his lips, beseeching silence after having been heckled throughout the race.

"Maybe I shouldn't have been so competitive in a race that I would have been slaughtered in if it were an all-men's field," says Mr. Cordes, a 48-year-old California real-estate broker, in an email. He was one of nearly 700 men among more than 12,700 women in the Nike Half Marathon.

This year, at least a dozen races styled for women are having their debuts across America, from the Nashville Women's Half Marathon on Sept. 25, to Long Island's Divas Half Marathon a week later.