Anatomy of the Perfect Workout Playlist

On Sunday, I'll be joining 45,000 other idiots in running the Chicago Marathon. In the months leading up to the Big Day, I've woken up early every Saturday to lace up my shoes, struggle into a sports bra and what can best be described as "running spanx", chug water and a few energy cubes, pop my headphones into my… » 10/11/13 1:50pm 10/11/13 1:50pm