The sense is: “Keep the covenant faithfully, and so shall your own and your children’s days be multiplied as long as the heaven covers the earth.” …

In the land which the Lord swore to your fathers to give them. — "It is not written here ‘to give you,’ but …

That your days may be multiplied
Long life being a very desirable blessing, and which is promised to those that obey…

Let all be directed by the three rules given here.