a long-distance race that is run on foot, particularly one that is 26 miles and 385 yards (42.2 kilometers) long;
e.g. She is training for the marathon that she plans to run next month.
(noun)
some undertaking, event, or situation that is difficult and that requires concentrated effort over a long period of time, thus constituting a contest of endurance;
e.g. He has a marathon of continuous meetings spread over the whole day. Pursuing this goal is a marathon that will require many months of significant work.