MeAm Lo'ez on Yeshayahu 40:30-31

The persecuting nations who relied on their own strength will eventually become weary, but the trusting Jewish people will be invigorated.

And do not complain about your sufferings in exile, for through them you were refined: the "young men" who nurture their physical bodies become weak when they reach old age, but those who nurture their souls and minds continue to grow throughout the physical weakness of old age.  They shall reach very great personal spiritual levels - they will "sprout wings like an eagle" - but will continue to "run and march" within society to edify it and will not tire of this great task.