And no, developers are not expected to be "in a sprint more or less at all times".
To the contrary -- in between each sprint, there's a review, a retrospective, often a weekend, then regrouping and feature analysis, planning poker, decision on which features to impement, and getting it up on the board.
That's the whole point. Instead of a long marathon without revision/reassessment/regrouping, it's a series of short sprints interspersed with revision/reassessment/regrouping. The metaphor here is the distance traveled, not the speed. Sprints are shorter distances, that's all.
Nobody has ever intended "sprint" to mean that engineers are going faster than in a marathon, anymore than "waterfall" means engineers are all tumbling down white water to their deaths at the bottom. :)