John Gill Commentary Deuteronomy 30:20

"to love Jehovah thy God, to obey his voice, and to cleave unto him; for he is thy life, and the length of thy days; that thou mayest dwell in the land which Jehovah sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give them." — Deuteronomy 30:20 (ASV)

That you may love the Lord your God
And show it by keeping his commands:

[and] that you may obey his voice ;
in his word, and by his prophets:

and that you may cleave unto him ;
and to his worship, and not follow after and serve other gods:

for he [is] your life, and the length of your days ;
the God of their lives, and the Father of their mercies; the giver of long life, and all the blessings of it; and which he had promised to those that were obedient, to him, and which they might expect:

that you may dwell in the land which the Lord sware unto your
fathers, to Abraham, to Isaac, and to Jacob, to give them ;
the land of Canaan, often thus described; this was the grand promise made to obedience to the law, and was typical of eternal life and happiness; which is had, not through man's obedience to the law, but through the obedience and righteousness of Christ.