According to most versions of the Mesoamerican model for Book of Mormon geography theory, the Isthmus of Tehuantepec (the narrow strip of land separating the Gulf of Mexico and the Pacific Ocean) constitutes the “narrow neck” of land described in the Book of Mormon (Alma 22:32).
While this strip of terra firma certainly reflects a narrow neck of land between two larger bodies of land, some may question how this section could be traversed in a day and a half by a Nephite when the narrowest point is still 137 miles across.
The Book of Mormon’s narrow neck was obviously not extremely narrow because in Mosiah 8, we read that the Limhi explorers unknowingly passed through it and thought they were still around the land of Zarahemla.
There are several possible explanations for the travel-distance dilemma. John Sorenson explains that travel distance for “a Nephite” might refer to someone accustomed to traveling long distances in short periods (as opposed to someone who was not “a Nephite”). Sorenson cites the example of some Mexican Indian foot runners who have been known to run up to 500 miles in six days, pausing one day for rest before running back again. Sorenson also cites the example of a man who, in 1973, walked 500 miles in six days as well as a man who traveled 161 miles in a single 24-hour period.
There may be a simpler answer, however. Our initial inclination when reading Alma 22:32 is to envision the narrow neck’s distance as a day and a half’s journey from ocean to ocean, but the verse actually says that a Nephite could make the roughly 36-hour journey from “the east” to the west sea (not the east sea to the west sea). It’s possible that “the east” referred to some other delineator other than the east sea — perhaps something much further inland (which could have greatly reduced the travel distance to the west sea).
Lawrence Poulsen, whom I think has the most up-to-date theories for the Mesoamerican model, argues that “the east” in Alma 22 refers to travel along the natural east border in the Isthmus of Tehuantpec and then true east along the northern border of the barrancas (bluffs) to the Cuatzacoalcos River. This river forms the eastern boundary of the Isthmus while the barrancas creates a natural border of cliffs that are over a thousand feet high as well as a thick wilderness.
Many years ago, Poulsen theorized — based on the description and geographical qualifiers given in the Book of Mormon — that this area could very likely be the Book of Mormon “Hermounts.” According to Alma 2:37-38, Hermounts “was that part of the wilderness which was infested by wild and ravenous beasts” and “many died in the wilderness of their wounds, and were devoured by those beasts.”
Poulsen’s Hermounts argument was part of a much larger argument and context — a portion of which identifies (like Sorenson) the Grijalva River as the Book of Mormon’s River Sidon. The Nephite culture may have lived in the Grijalva River basin. As Poulsen notes, “The (Isthmus) of Tehuantepec is (found) to the northwest of this basin and is (separated) from the Grijalva basin by a range of mountains on the eastern border of the (I)sthmus. This (mountainous) area called 'barrancas' is almost totally (uninhabited) even today.”
Several years after Poulsen suggested that the barrancas of the isthmus matched the Book of Mormon Hermounts, he discovered additional information that helped support his theory. The word “Tehuantepec” is a Nahuatl (early Mesoamerican) word composed of tecuani and tepec. While "tepec" translates as “hill” or “mountain,” "tecuani" means “man-eating beast.” Thus "Tehuantepec" means “the mountains of man-eating beasts.”
The Tehuantepec coat of arms, for example, has both a glyph for a mountain as well as a jaguar or "tecuani." While "tecuani" is typically translated as “jaguar,” an alternative translation is “man-eating beast.” According to Poulsen, Matt Roper — another LDS scholar — claims that natives will avoid that particular area in the isthmus for fear of man-eating beasts.
While Poulsen acknowledges that the Nahuatl language may have come late in this area of Mesoamerica, he correctly points out that when later cultures translate names into their own languages, they frequently adapt their translations to the names and features given by the former culture's language.
What does this interesting and Book-of-Mormon-favorable information have to do with the travel distance for a day-and-a-half’s journey for “a Nephite”? The isthmus’ eastern border — the barrancas that could easily be identified with Hermounts — is less than halfway across the isthmus. A Nephite traveling from the west sea (the Pacific Ocean side of the isthmus) to “the east” border (the natural east border in the isthmus) could easily cross that distance in a day and a half with little difficulty.