marathon

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cross-country footrace of 26 miles, 385 yards (42.195 kilometers).
• noun A long-distance race other than a footrace.
• noun A contest of endurance.
• noun An event or activity that requires prolonged effort, endurance, or attention.

from the GNU version of the Collaborative International Dictionary of English.

• noun A footrace of 26 miles 385 yards.
• noun Any long and arduous undertaking, straining the endurance of the participants.
• noun a battle in 490 BC in which the Athenians and their allies defeated the Persians.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A 42.195 kilometre (26 mile 385 yard) road race.
• noun figuratively, by extension Any extended or sustained activity.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a battle in 490 BC in which the Athenians and their allies defeated the Persians
• noun a footrace of 26 miles 385 yards
• noun any long and arduous undertaking