what did Israel need to do in order to enjoy the covenant promises?
Israel needed to come to God and believe what He wanted them to believe. The context states that if they had done so, they could have enjoyed all the covenant promises nationally. Their self-righteous attitude prevented it, so faith and humility, not self-righteousness, were required.
Les teaches that Israel was under the covenant promises nationally, and if they would have just come and believed what God wanted them to believe, they could have enjoyed all the promises. He illustrates this with the self-righteous brother in the parable, who insisted he had always served and been obedient; the father replied, "Son thou art ever with me, and all that I have is thine." Les explains that this fit Israel, but their self-righteous attitude kept them from enjoying the promises. So they needed to come and believe rather than trust in their own righteousness.