This is a merism that means the whole land over which Jehoshaphat reigned, from the most southern part to the most northern part, and is intended to include the whole land in between. Alternate translation: "of all the land of Judah" (See: [[rc://en/ta/man/translate/figs-merism]])
The author speaks of Jehoshaphat's leading the people back to serving Yahweh as if he were carrying them back to where they were at an earlier time. Alternate translation: "convinced them to return to Yahweh" (See: [[rc://en/ta/man/translate/figs-metaphor]])