The Collaborative International Dictionary of English v.0.48:

marathon \marathon\ n.
   1. A footrace of 26 miles 385 yards. [WordNet sense 2]
      [WordNet 1.5]

   2. Hence: Any long and arduous undertaking, straining the
      endurance of the participants. [WordNet sense 1]

   Syn: endurance contest.
        [WordNet 1.5]

   3. (Capitalized)a battle in 490 BC in which the Athenians and
      their allies defeated the Persians. [WordNet sense 3]

   Syn: battle of Marathon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

marathon
    n 1: any long and arduous undertaking [syn: marathon,
         endurance contest]
    2: a footrace of 26 miles 385 yards
    3: a battle in 490 BC in which the Athenians and their allies
       defeated the Persians [syn: Marathon, battle of Marathon]