Alma's account is explicit that the Lord forewarned Alma of Noah's army's approach (Mosiah 18:34; Mosiah 23:1). With such protection from the Lord, it is not surprising that he would also assist in the travel as well, as this verse indicates. The people of Alma, traveling with families and flocks, manage to outdistance a military expedition, or at least move sufficiently rapidly to an area that the army could not find them.
This protection in their flight is similar to the protection enjoyed by the Limhites in their flight before the Lamanite army. Alma's group had repented as part of their formation as a separate entity. By the time the Limhites fled Lehi-Nephi, they too had been humbled, and had repented and covenanted with God. Both of these groups found protection in the Lord.