God tells the prophet to speak, but the prophet doesn't know what to say. People are weak compared to the strength of God.
Note that it is not just the prophet who is supposed to be telling about the advent of God's presence. Zion and Jerusalem, that is all the faithful, are called to proclaim, and to do so loudly and visibly.
God is coming, mighty as an army, but not to destroy. God is coming to be our shepherd, to feed us, to carry us, to lead us.
Lectio Divina: Isaiah 40:10-11