Imagine you're a volunteer standing at the end of some marathon, eager to hand out medals, some guy swishes past the finish line, you try and give him a medal, he tries to punch you because he's just lost and come in 15th and he's about to lose his sweat-wicking visor endorsement contract, so you try to give a medal to the next guy, who does punch you, but since he totally fell apart in the last six miles he doesn't have much in his punch -- but at some point, some non-elite who's just had the best day of his life is going to cross that finish line and he's going to want his freaking finisher's medal.
So, how do races know when to start handing them out?