Geography: When Mosiah leads his people out of Nephi, he leads them into the wilderness. "The term 'wilderness' in the Book of Mormon apparently refers to mountainlands or forests as well as dense jungles. The term 'wilderness' means uninhabited areas." (Allen, Joseph L. Exploring the Lands of the Book of Mormon. Orem, Utah. SA Publishers. 1989, p. 287).
While there are many "wildernesses" in the Book of Mormon, we are particularly interested in one "wilderness" that is consistently described as a buffer between the land of Zarahemla and the land of Nephi:
" In late B.C. times a continuous wilderness strip separated Nephite Zarahemla from Lamanite territory. Furthermore, at least during the events recorded in the books of Mosiah and Alma, the city of Nephi (also called Lehi-Nephi) was some distance from the "narrow strip of wilderness" proper. On the Lamanite side of the border zone considerable wilderness space seems to have separated the city of Nephi from the transition strip. A good deal of searching for lost lands, marchings and countermarchings of foes, and wilderness travel went on in that extensive space. (See, for example, Mosiah 19:9-11, 18, 23, 28; 23:1-4, 25-31, 35; Alma 17:8-9, 13; 23:14, in light of verses 9-12; 24:1.) (Sorenson, John L. An Ancient American Setting for the Book of Mormon. FARMS. 1985, p. 12).
Sorenson suggests the mountain range along the North/Northwest border of the valley of Guatemala that separates that area for the Grijalva river valley, his location for Sidon (see Sorenson p. 153). Thus when Mosiah leads his people out of the land of Nephi, they head towards the general area of Zarahemla, passing through the "wilderness" which in this case is a mountainous strip.
Coming out of the "wilderness" the people of Mosiah are going "down" to Zarahemla. Sorenson has pointed out that in the pre-modern-map world, terms such as up and down have to do with elevation, and not cardinal direction (Sorenson 1985, p. 23). In the case of Zarahemla, it is consistently "down" from the land of Nephi (Allen, 1989, p. 289). This makes sense as Zarahemla is located along a river, and the river necessarily is in the lower elevations of the valley it courses through. Even so, highland Guatemala is yet a higher elevation, and the real world topography fits the consistent references given to it in the Book of Mormon.
History: We do not know how long the journey from Nephi to Zarahemla took. We do know that it was not necessarily a short journey. When Ammon leaves Zarahemla for the land of Nephi, it takes them 40 days (Mosiah 7:4) and Alma's people traveled the opposite direction in 21 days (Sorenson 1985, p. 8). Sorenson specifically notes that the area into which they went was a location where many appeared to "wander" through (Sorenson 1985, p. 12), as indicated by the widely different times for Ammon and Alma (especially considering that Ammon was with a small party, and Alma had both a large group of people and flocks with him).
With the variability of the time to travel the plausible 180 miles (according to Sorenson's reconstruction - see the map on p. 11) it would appear that Mosiah's group might have taken a significant amount of time. Note the way in which the journey is described: "And they were admonished continually by the word of God; and they were led by the power of his arm, through the wilderness until they came down into the land which is called the land of Zarahemla."
To be "admonished continually" suggests that this was not a short journey, but was long enough that the patience of the people might have been tried to the point where the Lord needed to "admonish" them. They were guided through the wilderness, but we must remember that Israel was also guided and supported by the arm of the Lord in their "wilderness," and their journey was also very long. We may suppose that Mosiah's people did not take the most direct route, but eventually found their way. It would not be out of the question to presume at least the 40 days Ammon's group required, and given the larger numbers of Mosiah's people, perhaps longer than that.
During their trek through the wilderness they would have had to subsist on the land, as they were fleeing and probably were not able to provision well for travel. As city dwellers, they would have been most familiar with agriculture, but we need remember only as far back as Enos to understand that the arts of hunting would not have been unknown to them. Indeed, many of those who tended the land might have supplemented their diet by hunting, and thus the party moving through the wilderness would not be without the ability to provide for that long journey.