According to E. L. Peay, if people "did travel to an exceedingly great distance" in the "forty and sixth year" (Helaman 3:3, 4), then it was nine years since the ship of Hagoth went into the land northward and many shiploads of supplies had apparently been delivered to them. Now there were other large groups of people moving out of the land of Zarahemla, apparently by land. Among these emigrants were the converted Lamanite people.
Some people must have scouted out the land northward and found it safe and fertile. Thus a large group of the people of Ammon migrated to that land. The people in Zarahemla knew where they went and how they fared, even though they went an "exceedingly great distance." We know there was contact with these emigrants because Mormon, hundreds of years later, knew what happened to these people who went into the land northward. He gave us an overview and told how they spread over the many years (Helaman 3:9-14). [E. L. Peay, The Lands of Zarahemla: Nephi's Land of Promise, pp. 105-106]