Clearly, Christ's expectation of the setting up of the Kingdom of God was not that it would come soon. He often said things like "Behold I come quickly" but the word "quickly" has changed it's meaning since the days when the King James Version was translated. Then it meant "in a lively way" or "suddenly". There is a saying in English, "the quick and the dead" which we think means if you are not fast to react to danger, you could end up dead. But that's not what the saying means. When it was first spoken, it meant, "The Living and the dead". The "quick "are the living not the "fast on their feet"!
Most of the Lord's parables about the Kingdom involved him being away for a long time and then returning. The operative principle is spoken by Christ himself in Acts 1:7 when the dsiciples asked him when the kingdom would be set up. He said, " It is not for you to know the times or the seasons which the Father has put in His own power." and Matthew 24:36 "But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only."
Regarding Mark 9, the word "come" is "parousia" which means "presence". Note that immediately after saying this, the disciples, Peter, James, and John went with Jesus to a high mountain apart by themselves. There they saw a vision of the glorified Christ in the company of Moses and Elijah. So, in essence, they saw the Kingdom of God in power in the vision thus fulfilling the statement in verse 1. See 2 Peter 1:16 when Peter talks about the transfiguration and says that they were "eyewitnesses of his majesty." They saw the Kingdom of God in power and glory. Others have had this same vision - indeed John did for a second time as he wrote the book of Revelation. One can think of Isaiah and the Apostle Paul too in his vision of the "Third Heaven" as recorded in 2 Corinthians 12:2 [compare 2 Peter 3 and note what the 3 heavens are in that chapter and compare 2 Peter 3:13 - New Heavens -that's the third one mentioned in the chapter - and compare Isaiah 65:17,18 - this new or third heaven is the Kingdom. Clearly that's what Paul saw just like the other senior disciples on the mount of tranfiguration; he saw a vision of Christ and the saints in glory.
When will the Kingdom be established? All the signs of the times point to the fact that, after so long, it will soon finally be here. See Ezekiel chapters 37 to 39; Luke 21 and Matthew 24. We are in the "end times". The Kingdom is coming and we must be ready for it when it comes in the glorious person of the Lord Jesus Christ! We echo the words of John at the end of the book of Revelation, "Even so, Come Lord Jesus!"
I hope you have found this helpful.