Yeah you're right, I hear it more like "this is a week long hike, not a sprint" as if a marathon included rest. In any length of racing there's no tomorrow. But I'm doing tongue-in-cheek pedanticness here and will stop that right now !

In a marathon, not sprinting is the rest.

I'd wager that if a manager says that they want you to take it more like a real marathon and less like long hike.