I know this to be true (Is 40:28), I certainly sing this truth but this is not always the same as letting the truth set me free.
I, unlike God, very much do grow weary. This has (and is) a testing time with developments at the shop, the recent and excellent Soul Purpose, last weekends Borderlands, working on a church merger, in addition to all the normal business involved in church leadership.
My own tendency is to get my head down and just work harder, to press through until I come out the other side. This fails on a number of levels but mostly because ministry becomes something I do and not something that God does through me. Weakness and tiredness reminds me that my ministry (such as it is) is not something to be built upon my strength or abilities (such as they are) but upon His grace, His goodness, His sovereign purposes and through the power of His Holy Spirit.
Of course it is perhaps harder to remember that when I’m not feeling tired, but that’s not how I’m feeling right now. Fortunately God does not grow weary, His power is not diminished by time or strenuous activity, His wisdom, knowledge and justice not compromised by a tired mind. Therefore He remains able to do all that He plans and wills, and as far as I’m concerned His grace is sufficient and He is enough for me.
And whether I am tired or not that is good to me.