Fitzgerald's first description of Myrtle makes us realize that Myrtle is very opposite from Tom's wife, Daisy. She is described as being in her middle thirties, and "faintly stout, but she carried her surplus flesh sensuously as some women can." She is wearing a navy blue dress and Nick tells us directly there was no "facet of gleam of beauty but there was an immediately perceptible vitality about her." This description in contrast to the cool sophistication of Daisy is immediately striking.
In terms of how she lives, she is married to "blond, spiritless man" who Tom says is "so dumb he doesn't know he is alive." They live in the apartment above a run-down, not prosperous garage. The garage and apartment are located in the valley of ashes -- a bleak and desolate strip of land between the Eggs and New York where the ashes were literally heaped and that resemble a kind of waste land. Myrtle can get away with this affair because Tom has created a reason to stop by with the promise to sell the car to Wilson. He is so desperate for the opportunity, he doesn't look beyond that to what is happening with his wife. She is excited to be in the affair with Tom because even though they are both married, she gets to escape the reality of the garage and live her pretend life with Tom in the apartment in the city for a little while. She gets to pretend to be someone she isn't. The misery of life with Wilson at the garage certainly plays a part in her willingness and ability to be in this affair with Tom.