An epic is a long poem or other work of art celebrating heroic feats. After you sail around the world for seven years, fighting corruption and planting vegetable gardens, some poet will surely write the epic of your adventures.
Epic comes from the Greek word for song, because Greek poets like Homer sang their poems. We tend to use epic for long, ambitious novels or movies, especially if they involve a long journey. Epic can be used as an adjective to describe something historically important, lasting and complex. Perhaps your great-grandfather was a soldier in the epic struggle of World War One. Your three day, sleepless card poker game could be described as epic.