When Lehi led his family from Jerusalem they left knowing that they would go, and knowing that they were heading into the wilderness. It may therefore be presumed that they brought what they would have needed for the trek. Nevertheless, in verse 11 Nephi notes that they did gather "all the remainder of our provisions which the Lord had given unto us." Nephi's precedent for this is unclear. The Lord provided the Liahona, but the text does not specifically mention any other items provided by the Lord. It is entirely possible that Nephi is referring to the items which they had brought with them, the accumulation of which was made possible by the Lord.
They are now preparing to leave the valley of Lemuel, which some writers have tentatively identified as Wadi El Atal on the Eastern coast of the Red Sea (Joseph L. Allen. Exploring the Lands of the Book of Mormon. SA Publishers, 1989, p. 264). If this were the location of the valley of Lemuel, they would have been approximately 200 miles from Jerusalem during their stay.
When they ready themselves to leave, they gather "seed of every kind." It is not certain where the seed came from. It may be that they had brought seed from Jerusalem, but it appears that they gathered seed from some location relatively close by the valley of Lemuel, as Nephi notes that they gathered the seed. From the time of the command to leave and the departure, there is clearly no time available for a quick trip back to Jerusalem. They took what they currently had, or could find in the vicinity, and prepared themselves for a long journey and permanent relocation.