Albert Barnes Commentary Joshua 13:1

"Now Joshua was old and well stricken in years; and Jehovah said unto him, Thou art old and well stricken in years, and there remaineth yet very much land to be possessed." — Joshua 13:1 (ASV)

Joshua is instructed to allot the whole of the promised land among the twelve tribes in faith that God would perfect in due time that expulsion of the Canaanites which Joshua himself could not carry further .