His lungs are already aching, from the sharp air and the breathless feeling that comes from dodging branches (though one thin one smacks him in the arm and he'll have a mark come evening but he doesn't give a damn in the slightest) and riding like you've got nothing to lose.

It's how he lives.

You run like hell or you get caught, and if you get caught, then you've got no reason to be running in the first place.

When they hit a narrow stretch, it's then and only then that Doc tightens up on the reins a bit, slowing him down. The gelding fights him on it, protesting the slowing to a canter with a loud snort and tossing of his head, angry at having to slow, even with his breath coming just as hard as Doc's is, thin sheen of sweat broken out over that dark coat.