The Starfleet Academy marathon is a race of Starfleet cadets that occurs once a year.
In the marathon of 2323, on the planet Danula II, Jean-Luc Picard became the first freshman cadet to win the marathon. (TNG: "The Best of Both Worlds, Part II")
Background
The certificate given to Cadet Picard for winning the marathon
A certificate created for the Picard family album in Star Trek Generations, which was not seen onscreen, stated that Picard won the forty first academy marathon in April of 2323. It also states that this marathon is over 40 kilometers, although the accepted distance since 1921 has been 42.195 km.
It is possible that the reason why Picard's academy picture in Star Trek Nemesis depicted him with a shaved head is because he shaved his head for this marathon.