This is a scripture of encouragement. Even though as each day passes we get a little older, we feel a little more tired, we are more set upon by the circumstances and pressures of the world, inside we are growing.
We're growing spiritually - in our faith, in our trust in God, in our knowledge, in our capacity to love and in our ability to discern God's movement in our lives. Therefore, the fact that our earthly body may be deteriorating doesn't matter because this is just a prelude to the joy, peace and love to come when we join together with God and our family in Christ in heaven.
Another facet of this is that no matter how tired or defeated we may feel - emotionally, mentally or physically - we need to remember that it is our trust in Him that enables us to be strengthened and sustained through whatever we are going through in life, each and every day.
Be encouraged because we are growing in our faith, trusting God more, and developing into the people God intended us to become.