A marathon is 42.195 kilometres long - about the same as running from one end of a city to the other. For most adults, finishing one is a major achievement. Eliud Kipchoge ran one in 1 hour and 59 minutes and 40 seconds. That is a pace of about 21 km/h, faster than most people cycle.
Many of Kenya's best runners come from a community called the Kalenjin. Scientists think there are several reasons: they grow up at high altitude (where the air is thinner, so the body learns to use oxygen well), they often run several kilometres to school each day from a young age, and there is a strong culture of training together.
The little town of Iten in Kenya's Rift Valley is so famous for producing marathon champions that runners from all over the world travel there to train. The road signs at the edge of town call it 'the home of champions'.
Running is not just about being fast. It is about being patient, training every day, and not giving up when it gets hard. Kenyan coaches often say the body learns to run, but the mind learns to keep going.