“Every place that the sole of your feet shall tread on, to you I have given it, as I said to Moses.”
The land was to be theirs, but it had to be possessed. Step by step they would receive it as they went forward by faith in YHWH. Sometimes it would be two steps forward and one step back, but always they should go onwards until the whole was theirs. For once they had trodden it, it belonged to them. And all this was in accordance with His promise to Moses. Moses may be dead but God had not forgotten Moses, and He had not forgotten His promises to him. They still stood firm.
These verses (Joshua 1:3-5) echo the words of Moses in Deuteronomy 11:24-25. There too possession would depend on going forward in obedience to YHWH.
We too must remember that those who would accomplish things in God's name must be prepared to go forward step by step. As we do so He will lead us in the way (Genesis 24:27) and grant us our part in His work.