The Kingdom of God is not a place, of course, but a condition. Kingship might be a better word. "Thy kingdom come, thy will be done," Jesus prayed. The two are in apposition.
Insofar as here and there, and now and then, God's kingly will is being done in various odd ways among us even at this moment, the Kingdom has come already.
Insofar as all the odd ways we do God's will at this moment are at best half-baked and halfhearted, the Kingdom is still a long way off-a hell of a long way off, to be more precise and theological.
As a poet, Jesus is maybe at his best in describing the feeling you get when you glimpse the Thing Itself-the kingship of the king official at last and all the world his coronation. It's like finding a million dollars in a field, he says, or a jewel worth a king's ransom. It's like finding something you hated to lose and thought you'd never find again-an old keepsake, a stray sheep, a missing child. When the Kingdom really comes, it's as if the thing you lost and thought you'd never find again is yourself.