The Bible addresses strength in hard times directly in several passages:
Isaiah 40:29-31 (KJV)
29 He giveth power to the faint; and to them that have no might he increaseth strength. 30 Even the youths shall faint and be weary, and the young men shall utterly fall: 31 But they that wait upon the LORD shall renew their strength; they shall mount up with wings as eagles; they shall run, and not be weary; and they shall walk, and not faint.
Philippians 4:13 (KJV)
13 I can do all things through Christ which strengtheneth me.
Psalms 46:1 (KJV)
1 God is our refuge and strength, a very present help in trouble.
2 Corinthians 12:9-10 (KJV)
9 And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. 10 Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ’s sake: for when I am weak, then am I strong.