Chances are your last half marathon wasn’t exactly 13.1 miles long. In fact, it’s likely your GPS said it was just a smidge over that: 13.4, perhaps, or maybe even more than 14 miles. It happens – a lot. By that logic, then, most courses are measured inaccurately…right? Avoid putting too much stock in what your GPS watch says, cautions coach Mario Fraioli, founder of The Morning Shakeout newsletter and podcast.