It is impossible to get exhausted in work for God. We get exhausted because we try to do God's work in our own way.
Oswald Chambersin the will of God" is not a matter of intellectual discernment, but a state of heart... It's motto is --" My Father can do what he likes with me, He may bless me to death, or give me a bitter cup; I delight to do His will.
Oswald Chambers