In the Boston marathon, only the elite runners (a few dozen) are segmented by gender/sex, and elite women are the first to leave, followed by the elite men, followed by the top qualifying times of either gender.
From watching the race along the road yesterday, there were no elite men who caught the main pack of women (though a few straggling women did fall back into the elite male pack). Following that, while it was mostly a male non-elite pack at the beginning, there were a few women scattered throughout, and then it was fairly blended about 15 minutes later.
So while they stagger men and women to prevent interference in the race at the very highest levels (and a woman crosses the finish line first), women below that tier have a ton of opportunity to push themselves against men.
Edit: Also, they do track overall finishers, time-wise, regardless of gender. So a woman might see that she was the 4th finisher in her division (young women), and 16th overall.