The bulk of Isaiah 40 is devoted to showing our incomparable God (Isaiah 40:12-26).  The description of our God makes our question in Isaiah 40:27 inconceivable.  Of course God knows how difficult our way is in a world where the godless are in power!  Then, after restating God’s resources of strength and knowledge (Isaiah 40:28), we learn that all His power is transferable to those who “wait” for Him (cf. Isaiah 40:29-31).  Our Lord continues to save His people because His strength is added to our weakness so we keep putting on fresh strength each day.  Instead of being overcome with circumstances, God stabilizes our emotions and spirits.  So, is it true or false when a Christian says in frustration: “I can’t take it anymore!”?  And, of course, we know that all this available strength came as a result of Jesus’ experience of weakness on the cross.  He experienced ultimate weakness so we could experience ultimate strength as we trust Him.