John Gill Commentary Deuteronomy 11:9

"and that ye may prolong your days in the land, which Jehovah sware unto your fathers to give unto them and to their seed, a land flowing with milk and honey." — Deuteronomy 11:9 (ASV)

And that you may prolong your days in the land
Not only enter it, and take possession of it, but continue in it long, which depended upon their obedience to the laws of God:

which the Lord swore unto your fathers to give unto them and to their
seed ;
had promised with an oath, so that they might be assured of the enjoyment of it, though they could not be of their continuance in it, unless they obeyed the divine commands:

a land that flows with milk and honey ;
abounds with all good things, whose fruits are fat as milk, and sweet as honey; so the Targum of Jonathan.