There are two ways to read this statement. One is that “them” refers to the Nephites, and it that the Nephites were in such panicked retreat that they could not be stopped until they came to Jashon. This reading would fit if we accept the cowardly definition of the Nephite military at this point.
The second reading is that “them” is the Lamanites, and that the fleeing Nephite army was unable to find a way to deter the Lamanite invasion until the battlefield arrived at Jashon. This is the more likely reading, as the Nephites did stop at Jershon, and were able to form a sufficient defensive line that the Lamanite threat was blunted to the point that Mormon could take the time to go to the land Antum where the plates had been deposited (next verse and Mormon 1:3). If the Nephite army had been in complete panicked flight, one of the most likely scenarios would have massive dissertions where the army would fission into pieces and flee in different directions.
If the army were attempting to avoid engagement, desertion is a more likely scenario. This does not happen. They stay together and become at least temporarily effective at Jershon. The relative probabilities of turning a discispline retreating force, and an undisciplined fleeing force to make an effective stand tells us that this is not the panicked flight it might appear to be.
Geographical: There is no specific geographical information available for Jashon other than to indicate that it is along the northward flight path. The presence of a city in that location does indicate that it is a populated area as opposed to some type of wilderness. ((John L. Sorenson, The Geography of the Book of Mormon. FARMS, 1990, p. 313-4). This entire geographic sequence does not fit into previously known names and lands. This might be because of the renaming of locations, but it is also quite likely that this land experiences the general population explosion at the beginning of the Classic period in Mesoamerica. Since Mormon gives us little of the intervening time periods, new settlements surely arose during that time. We are probably seeing the result of the last three hundred years’ growth in the area.
Chronological: The three hundred and forty fifth year would be 335 A.D. At this time Mormon is probably twenty four chonologically, even though he is in the twenty fifth year from the year of his birth. The reason for the presumption is that he was told to obtain the plates when he was twenty four, and it was likely that, as Joseph Smith would be later, Mormon was anxious to receive them and would not unnecessarily delay. Of course the fact that they were in active flight from the invading Lamanites and Gadiantons played a role in dictating available time as well.