When I was a pioneer I used to talk to Christians who would tell me that the Kingdom of God is in your heart. Then, I would show them all the scriptures and prophecies that showed the Kingdom to be literal, here on earth.
It wasn't until I became a Christian and did a study on the Kingdom Parables that everything really fell into place. Like a lot of things that people argue about - the correct understanding isn't an either/or, but rather a both.
In a nut shell: The Kingdom of God exists in its mystery form during the church age when the Holy Spirit indwells believers.
To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory: - Col. 1: 27
The body of a believer is the true temple of God. God desires to inhabit people. But, when the Millenial Reign begins after the Great Tribulation, Jesus will indeed rule the earth in a literal kingdom from modern day Israel in a rebuilt temple.
It will take some time, but going through the meaning of the kingdom parables in a methodical study in a traditional Baptist Church is very illuminating. .
Studies on the Kingdom Parables.