I think this speaks of His spiritual Kingdom. The physical kingdom, where He will rule with a "rod of iron" comes with His next coming.
Luke 17:20 ¶ And when he was demanded of the Pharisees, when the kingdom of God should come, he answered them and said, The kingdom of God cometh not with observation:
21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.
John 14:20 At that day ye shall know that I am in my Father, and ye in me, and I in you.
Revelations 12:5 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.
Revelations 20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
1 Corinthians 6:2 Do ye not know that the saints shall judge the world? and if the world shall be judged by you, are ye unworthy to judge the smallest matters?
3 Know ye not that we shall judge angels? how much more things that pertain to this life?
I have also heard some speculation that Jesus spoke of His transfiguration, resurrection, and the day of Penecost in this verse.