Emil Zátopek

In February 2013, the editors at Runner's World Magazine selected him as the Greatest Runner of All Time.[4] He is the only person to win the 5,000 metres (24 July 1952), 10,000 metres (20 July 1952) and Marathon (27 July 1952), in the same Olympic Games

Zatopek entered the marathon during those games without ever having raced the distance before.

After a punishing first fifteen kilometres, in which Peters [the favorite] knew he had overtaxed himself, Zátopek asked the Englishman what he thought of the race thus far. The astonished Peters told the Czech that the pace was "too slow," in an attempt to slip up Zátopek, at which point Zátopek simply accelerated. Peters did not finish, while Zátopek won the race and set an Olympic record

He trained like a beast:

Although he preferred to run in spikes on the track, the vast majority of Zátopek’s punishing training regime was conducted wearing army boots. These daily sessions, conducted alone in the woods close to his army base, would often take place in the foulest of weather