full, half

prepare for, train for
compete in, run, take part in
complete, finish
win

champion (esp. BrE), runner
running
He says that he keeps fit by marathon running.

12-hour, 700-mile, all-night, etc.
car, cycling (esp. BrE), etc.

journey, walk
effort (esp. BrE)
session
The painting would be completed in one marathon session.

match, race
a marathon five-set tennis match