The kingdom of God already exists here on earth because it is within the true believers in Jesus who make up His Kingdom here on earth where His will must be done as it is done in Heaven.
We choose and live in the Kingdom of God here on earth waiting for the real Kingdom to be established at the appointed time, after the end of the world.
1 John 5:4-5 ASV
(4) For whatsoever is begotten of God overcometh the world: and this is the victory that hath overcome the world, even our faith.
(5) And who is he that overcometh the world, but he that believeth that Jesus is the Son of God?