Geography: Alma’s answer to Zoram contains useful geographical information, first by identifying the Lamanites’ route. They were north of Zarahemla, or downstream. They must have retreated to the wilderness from which they had come to avoid the Nephite cities on the west which would have attacked them, hampering their retreat with captives. Logically, then, they were traveling south through the west wilderness.
The information that they would cross the Sidon south near Manti puts them on the more frequently used passage between the land of Nephi and the land of Zarahemla. Thus, I suggest that the way through the wilderness was a relatively difficult passage. Laden with captives and, no doubt, booty, the Lamanites opted for the easier route home.
Understanding this geography tells us how the Nephites, although unable to respond quickly enough to save Ammonihah, could still intercept the Lamanite army before it was completely out of Nephite territory. The Lamanite army was northwest of Zarahemla and had to travel along the coast for quite a distance before they were parallel to Zarahemla. Thus, the Nephite army could intercept them by traveling the shorter distance south. Knowing where the Lamanite army would be further decreased the required travel time, allowing for the interception near Manti.