According to Wikipedia,
A half marathon is a road running event of 21,097.5 meters, or
13+7⁄64 miles, about 13.1 miles. It is half the distance of a
marathon and usually run on roads. It is a popular race for runners
of all abilities, long enough to be a serious challenge for most
people, but not requiring the level of training of a marathon. In
consequence, many towns and cities hold annual all-comers half
marathon races through their streets. It is also common for a half
marathon event to be held concurrently with a marathon, using
almost the same course with an early finish.