It is stated in Helaman 4:7 that the Nephites "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." We could take the phrase "a day's journey for a Nephite" a number of ways:
(1) The distance could be related to depth. In other words, in those areas where the Nephites had stationed their armies, it would take a Nephite a day to travel from the front lines to the rear.
(2) The whole extent of Nephite fortification could have been limited to a distance starting from the west sea--and extending one days journey eastward.
(3) If the entire land of Bountiful stretched from the west sea across the narrow neck to the east sea, that distance could amount to just one day.
(4) Perhaps the "day's journey" refers only to the strategically fortified parts of the fortification line (for example: passable territory between mountains, or rivers, or swamps).
The reader should keep in mind that there is not any location on the continent which extends from a west coast to an east coast that only requires "one day's journey." [Alan C. Miner, Personal Notes] [See Geographical Theory Maps]
Geographical [Theory Map]: Helaman 4:6-7 Moronihah's Fortification Line (58th Year)