The verse arrives as an answer to a complaint. Israel has been saying my way is hid from the Lord, and my judgment is passed over from my God — the sense that God has stopped paying attention to their case. Isaiah's reply is not sentimental. He points at the size of God: the one who measured the waters, who calls the stars by name, who fainteth not, neither is weary.
Then the promise: they that wait upon the Lord shall renew their strength. The word for wait carries the sense of binding together, twisting like the strands of a rope. It is not passive. It is attachment — hanging on when the evidence is thin.
The order of the images is odd and often noticed. Mount up with wings as eagles, then run, then walk. That looks like a decline. But it may be exactly right. The soaring seasons are rare. The running seasons are demanding but finite. Most of a life is the walking, and the hardest promise to believe is the last one: they shall walk, and not faint.
Isaiah is honest that even the young get tired — the youths shall faint and be weary. Natural strength runs out on schedule regardless of how much of it you started with. The strength on offer here is not yours renewed. It is exchanged.