In our society strength is measured more in terms of physical strength. We gain this physical strength by our own means. But God is not talking about physical strength in Isaiah 40:28-31 or in Joshua 1:7-9. Our first reaction to the word strength is to imagine this hulking person who can tear phone books in half, but God sees strength a lot differently. The strength he talks about is an inner strength, an area in all of our lives that is under-developed. This inner strength... what is it used for? It is used to resist evil, it is used to get you through difficult times in your life, it is used to share God's word... when you don't want to or don't think that you can, it is used to do right. And the interesting thing about this inner strength is... it's not something we can "produce" ourselves. Whereas we can work out as much as we want to develop the outward, physical strength, the inner strength that God is talking about comes only from Him. See Isaiah 40:31, But those who wait on the Lord shall renew their strength... as far as I can tell there isn't any other way of being "renewed in strength" outside of waiting on God (making God a part of your entire day). In Joshua 1:7-9 the strength we "receive" from God is, again, not found by us... it is given to us when we stay in the Word, and "do not turn from it to the right or to the left." God wants us to be strong in Him, not in ourselves. The stronger we are in Him the more we please Him. And when we are strong in him we "do not have to be dismayed or afraid, because the Lord our God is with us wherever we go."

Isaiah 40:28-31