According to the Mary Sue, Meredith Fitzmaurice took a wrong turn during the Run for Heroes Half-Marathon and accidentally ended up on the full marathon track.
Instead of turning around and finishing her heat, like most of us would do, she thought she might as well finish the damn thing, and ended up winning the Women's Division. She was the tenth runner to cross the finish line.
She says that she decided to keep going when officials confirmed that she was on the marathon track, and when they told her that her time could qualify for Boston. Now I'm not a runner, so maybe this is a thing fitness-types do, where they just suddenly double their workout and win the whole freaking race, but damn.