Mormon finishing his record of the destruction of the Nephites (v. 1) seems to refer to the last historical event (see Mormon 6:15 below). In the remainder of chapter 6, Mormon laments over his people. Mormon 7 speaks to the surviving remnants of the latter days.
The Nephites marching forth “before the Lamanites” suggests they are being driven (v. 1). In the year A.D. 380, they began to flee from the Lamanites (see Mormon 5:6–7 above). How far and for how long they had fled or were driven, we do not know. It could have been a great distance. Cumorah was the agreed upon place to gather for the last battle (Mormon 6:2–3). The many waters, rivers, and fountains may have been for a strategy advantage to the Nephites, but it could have also been just a last attempt to end their flight (v. 4). How the events of the years A.D. 380 through 384 were divided is not given, but five years is a long time to be on the run. How long it took Mormon to gather all his people would not seem to have been lengthy, since whoever did not flee was destroyed (Mormon 5:7).