That term is full of meaning.
Luke used the it 33 times in his gospel.
(the kingdom of God is within [among] you. Luke 17:21)
(But the kingdom of the Father is spread out upon the earth, and men do not see it. Thomas 113)
It would certainly be well to try to get a handle on it.
Within and among are two different concepts, but at my time of life within becomes a paramount concern.
"Thy kingdom come": the first petition of Jesus, speaking to others.
How does the kingdom come in my life? Purity of Heart is to will one thing; all the little fishes go back.
Easier said than done. Our life is made up of the 'little fishes'. Give them all up? Is that what God wants? That's a deep question: to give God time means to sacrifice some of the wonderful activities that fill my life. Become a non person?
Years ago I worked as a Probation Officer, which involved a considerable amount of driving around the county. I had been trying to 'give God more time', learning quite a few of the Psalms and quite a few hymns. I found I could keep my mind on them to the exclusion of everything else. But I asked myself the question: is that what you want? And the answer of course was no; I was only 47.
Is there a happy medium, a balance between the inner and outer? What do you think?