"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; they shall walk and not faint." (Isa.40:29-31)
Forgive my lengthy opening text, but we needed it. There is something in these verses for young and old alike. As it says, even the young and most fit among us are not immune from periods of sheer exhaustion. But there is more than one kind of strength, and in today's world, it appears to me that moral strength can wane before physical strength has petered out, and with far greater consequences.
When Joseph outran Potiphar's seductive wife (Gen.39), it was not physical strength that gave him the edge, but moral muscle. On the other hand, Samson had enough stamina to slay a thousand men with the "jawbone of an ass" (Judg.15:15); but he didn't have enough moral fortitude to withstand the pressure of a whining woman. Moral strength can pick up when physical potency is running low to carry one beyond what seems humanly possible. But when moral integrity has been allowed to grow flabby, no amount of physical vitality can take up the slack. That's why, spiritually speaking, it is possible, as the verse says, for older people to walk, run, and even "mount up with wings as eagles," when young folks around them have grown weary, become faint, and fallen.
The secret to both physical and moral stamina is "strength training," something widely touted in today's body-conscious society. We should strengthen our bodies, by all means, but if we're more faithful to physical exercise than Biblical exhortation, it is easy to see which part of our lives will be stronger. And make no mistake; there are times when no amount of strong will and determination will win the victory. Only those who are spiritually fit can hope to overcome; and only those with well developed moral muscle will be spiritually fit.