In run down gyms, on the backstreets of Havana, boxers will spend endless hours skipping, and running, and sweating. They train for the chance to match against someone else who has spent as many hours doing the same.
Adversaries for the duration of the bout, friends the moment the bell sounds to end it, the result of each match is rarely as important as the effort each boxer has demonstrated. In one of the few places left where sport remains pure, they are driven by nothing more than their determination, as they trade blows watched only by their fellow boxers.
But the biggest challenge still lies ahead for the boxers of Havana. As Cuba gradually opens up - in a world where so many of our sports have been tainted by temptations of money - let us hope these boxers can stay true to their sport.
That will surely prove to be the hardest fight of all.