N-no, it's not that.

[He sounds extremely out of breath for a boy who just ran for ten seconds. He places a hand over his chest, because it hurts to breathe now, and his heart feels like it might stutter and fail if he doesn't hold onto it. Ahh.]

I have a rather weak body, so things like physical activity can be difficult for me to keep up with. [He coughs.] I suppose being out here... for so long didn't help my stamina, either, but it's just that my lungs won't cooperate.