“ Only be strong, and very courageous, that you may observe to do according to all the law which Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go.”
Here the strength and courage is related to the keeping of God's Instruction, ‘the Law'. He was not only to be strong and courageous in battle but also in life. He was strictly to observe God's moral law. Obedience was more important than physical strength and physical courage, although it would enable him in both. But failure in obedience would mean that it did not matter whether he was strong in any other way or not.
“Observe to do.” See Deuteronomy 5:1; Deuteronomy 5:32; and regularly in Deuteronomy (fourteen times). It is something that requires hard work and deliberate and constant attention and determination. It will not just happen. It requires careful study of the word of God and a heart fully responsive to God.
“Do not turn from it to the right hand or to the left, that you may have good success wherever you go.” See Deuteronomy 5:32; Deuteronomy 17:11; Deuteronomy 17:20. Success would depend on strict conformity to the will of God. Indeed that would guarantee success. But treating God's law lightly and deviating from it one way or another would result in disaster, for God would no longer act for him.
“Turn not from it.” The ‘it' is masculine and has in mind the law thought of as ‘the book of the law' (law is feminine). However LXX omits ‘law', and the ‘it' therefore there refers to what Moses had commanded. It may be that that was the original Hebrew reading, but it is more probable that it is simply LXX correcting a seeming difficulty which it does regularly.