I used to think getting faster was just about training harder. Turns out performance is capped by three separate systems: cardiovascular, muscular, and the mind, and figuring out which one is holding you back changes everything about how you train.

I flew from Berlin to Chicago two weeks after running my first sub-three-hour marathon, slept three or four hours a night, forgot my nipple tape, and still ran another PB. Here's how the Chicago Marathon went, from the starting corrals to the finish line topless.