Graham as much as anyone knows the source of his success. When He calls us to a task, He also equips us to do it. This eliminates any limitations we might think we have in life. In Christ, we have been unlimited resources. Whatever God commissions to do, we can do, because God's resources are available to us.
Batteries go dead.
We get tired. Machines wear out. So we conclude that we are no different. Our attitude predicts our failure. And I cannot permanently change my exercise habits.