He's Running a Marathon...
From Halopedia, the Halo wiki
He's Running a Marathon... is an achievement in Halo: The Master Chief Collection. It is unlocked by viewing the Half-Naked Guy Easter egg in the opening cutscene of Gravemind. The Easter egg only appears while playing on Legendary difficulty with the graphics switched to Classic mode. If the graphics are set to Remastered mode Beamish will appear instead, and the achievement will not unlock.
The achievement is worth 5 Gamerscore.