“Long ago I made it and I formed it in ancient times.” What God is saying is that “long ago” He made and formed the principles that are the basis of the events that occur in life—in this case the principles and covenants that led to the trouble that Judah was in. God is not saying here that long ages before Hezekiah’s time He decided that the Assyrian army would lay waste many cities in Judah. If that was what He was saying, then why would He have sent prophets to tell the Judeans to repent and obey His commands? If He had already decided ages before that the Assyrians would destroy Judah (and Israel), then sending prophets to get the people to repent would have just been some sort of pretend game. For God to send a prophet to tell the people to repent when He already knew they would not repent would only be some kind of immature “I told you so” one-upmanship.
God was sincere when He sent the prophets to the people of Israel and Judah, and those people had a genuine chance to repent and not only avoid destruction but to live and prosper. But when God’s people rejected Him and the laws He gave them, then the principles that God had set in place long before influenced what happened on earth and in this case the Judeans suffered at the hands of the Assyrians.