The term 'lengthy undertaking' can be described by two crossword solutions: 'MARATHON' and 'ODYSSEY'. A MARATHON is a long-distance running event, usually covering a distance of 42.195 kilometers (26.219 miles). It is named after the legendary run of the Greek soldier Pheidippides, who ran from MARATHON to Athens in 490 BC to deliver news of a military victory. An ODYSSEY is an epic journey, usually involving a hero or group of heroes. The term is derived from Homer's epic poem, The ODYSSEY, which follows the ten-year journey of Odysseus as he attempts to return home to Ithaca after the Trojan War. Both terms are used to describe a long and difficult journey, often with a heroic element.