A myth is a long narrative tale about heroes and their daring quests. A legend on the other hand, is a tale of how something came to be.
Source: My English teacher.
Myth is much longer than legends and subsequently have more characters. Often, these myths are intertwined and form a “mythology”. Like the Greek Mythology or Egyptian Mythology. Most often than not, they involve Gods and interactions with man but focus on a “Hero”.
Legends are shorter. They can be about the creation of anything: from mountains, seas, to fruits. Or maybe even the legend of a “tradition” or a place.
Remember, myths are about people or deities. Legends are about things.