He giveth power to the faint; and to them that have no might he increaseth strength.
Even the youths shall faint and be weary, and the young men shall utterly fall:
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. (Isiah 40:29-31, emphasis mine)
It is interesting and encouraging to remember that it is not by my strength, but by His that I perform the task laid out before me. Indeed, if it were by my strength I would have failed long ago. But in Him I have the grace and strength necessary to finish the race. After all, my God has already won. My victory hung beaten and bruised on that cross at Calvary, and was laid to rest in a tomb. But even death could not keep Him! Three days later He rose. Victory is mine - and yours - in Christ alone!
Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us,
Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God. (Hebrews 12:1-2)