In the 345th year (Mormon 2:16) the Nephites retreated first to the land of Jashon, and then "northward to the land which was called Shem" (Mormon 2:20). Assuming a Mesoamerican setting and according to John Sorenson, the distance to the land of Shem might have only been a single retreat sequence away from Jashon, and thus might have only stretched anywhere from 10 to 60 miles (Source Book, p. 301).
According to John Sorenson, the most likely Mesoamerican location for the land of Jashon and the land of Shem, although no geographical details are provided, would be on the strip around Acayucan and Hueyapan, south and west of the Tuxtlas [in Veracruz, Mexico], or else in the 50-mile sector from San Juan Evangelista in the direction of Tuxtepec. All these places were in the eastern, lowland sector of the land northward. According to the text's account of the final wars, there are no "ups" nor "downs" in the geography of the Nephite lands north of Bountiful. [John L. Sorenson, An Ancient American Setting For the Book of Mormon, p. 344] [See Geographical Theory Maps]
Geographical [Theory Map]: Mormon 2:20 The Nephites Are Driven Northward to the Land of Shem (345 A.S.--346 A.S.)