* δρόμευς (dromeus): This is probably the closest single word. It refers to a runner, often in a race or contest.
* θεατής (theates): While primarily meaning "spectator," in some contexts it could refer to a participant in a footrace, particularly in the context of ancient Greek games.
The word you choose will depend on the specific situation. For a general "runner," δρόμευς (dromeus) is the safest bet.