Verse 32 may be the most well-known geographic reference in the Book of Mormon. There was “a small neck of land between the land northward and the land southward.” That definition is also given as the line between Desolation and Bountiful. Thus, this narrow neck is the northern reach of Nephite lands until the ending years when the Nephites are pushed north into the land northward, or Desolation. The two names may be seen as representing the physical direction (northward) and the spiritual state (Desolation).
The narrow neck is only “the distance of a day and a half’s journey for a Nephite.” What that means is not precise. It is also difficult to interpret in light of Helaman 4:7 remarking that “And there they did fortify against the Lamanites, from the west sea, even unto the east; it being a day’s journey for a Nephite, on the line which they had fortified and stationed their armies to defend their north country”. It is possible that the difference is that one is the defended line, and the longer distance measures something more than the defended line.
Also of note is that neither of the two references (verse 32 and Helaman 4:7) says that the line is from sea to sea, even though that might be implied. In both statements, the west sea is mentioned, but some other point on the east boundary does not have a sea associated with it. It is possible that the distance to measure begins at or near the west sea, but that it does not extend all the way to the east sea. That would suggest that there was some impassible part of the small neck on the east, between the measured line and the east sea.