The previous verse indicates that God challenges his followers to grow in their Christian faith. Verse 12 suggests that as part of that challenge, our hands will grow tired and our legs will shake so much we think they will give out. In other words, we might want to quit. We must instead renew ourselves, find new strength, and then help those who still struggle. 

The previous verses let us know that God chastens (corrects/disciplines) out of love and not because He's on His last nerve. Verse 11 says that correction is always unpleasant at the moment. At that particular moment, because feelings are hurt, we are tempted to give in to the hurt instead of praising God that we not only know the correct thing to do but that He just reminded us of His love. So verse 12 says for us to rise up from the mully-grubs and get out of your pity party and praise God for His lovingkindness. The author is trying to keep up focused on the right thing instead of the natural and "that's human nature" thing.