In "The Letter of Marque," Patrick O'Brian uses a vivid comparison to convey the exhaustion and determination of a character. The quote refers to Plutarch's account of the heroic runner from Marathon to Athens, who delivered news of victory but collapsed from fatigue. By likening his sleep to that of the runner, the character indicates a profound state of weary resilience, suggesting that he is so fatigued that he can only rest once he has reached a breaking point. The imagery in this comparison paints a powerful picture of the mental and physical toll that relentless pursuit can take on a person. O'Brian effectively draws a parallel between the historical figure's unwavering commitment and the character's own struggle, showcasing themes of endurance and the cost of ambition. This line reinforces the novel's exploration of the sacrifices made in the pursuit of goal and honor.