A belief kept alive for forty-five years does more for a man's strength than the forty-five years take away.
Certainty compounds: every year a man rehearses a word he was given, he adds references to it, the thickened belief holds a state, and that state keeps a body doing at eighty-five what most abandon at fifty.
Choosing the hill country over the easy valleys is what that state looks like from outside; strength moves toward difficulty when the difficulty is where the promise was placed. And his certainty rests on what Yahweh said rather than on any estimate of his own powers, which is why he can allow that it may be and still ask for the worst ground on the map. Whoever cannot hold a belief that long inherits whatever the climbers left behind.