It could mean that the speaker has given up on being a 'hero' in the context of a novel a long time ago. Maybe in the story, the character was once a heroic figure but for some reasons, like being disillusioned or facing too many difficulties, decided to quit that role.
Perhaps 'hero' here refers to the idea of the main character or a heroic archetype in a particular novel. 'I quit a long time ago' might imply that the person has distanced themselves from the story or the concept of that hero for a significant period. It could be due to a change in personal interests or a feeling that the hero's journey in the novel was no longer appealing.