Today I think we misunderstand it too. We think of the kingdom as far away, in the future, and almost in attainable. When the children of Israel were in the wilderness they were surrounded by God's glory at their core. He came in a cloud by day and a pillar by night. He rested over the mountain or over the temple. He spoke like thunder and shook the earth with His power. He met Abraham at his tent to tell him of his soon son Isaac. His chariot of angels and fire took Elijah to the Heavens as Elisha was the only witness. He walked with Enoch and showed His back to Moses for the glory of His face would have killed him. He came as a baby into the womb of a young girl. Lived in a rough town and worked as a carpenter with Joseph. He gathered 12 men and gained a crowd. He debated with closed minded men and healed the hopeless. He forgave the weak and blessed the cursed. God has been pretty much everywhere in our human history.
And we want to say "I can't wait for Jesus to come again" or "where is God?" Don't get me wrong, it's good to want Him to come again but if that is all you are going to focus on you might miss out on what is right in front of you. He said that when you visit those that are imprisoned you are visiting Him. Feed the hungry, clothe the naked, and listen, you do those things to God.
When God said the kingdom of Heaven was at hand, He was talking about His presence WITH them. He meant the effect His glory would have RIGHT THEN. When He told His disciples that they would see His kingdom before their death it was true. His glory came to accomplish. In doing so 3 of His disciples had the privilege to see a strong portion of God's glory on a mountain. Jesus' face shown, His robe was so white that no other white could compare. Elijah and Moses met with Him, two men transported to Heaven previously. They consoled Him before His death. All of this within God's cloud brought to earth once again.
It's funny though. As God's glory shown down on the earth the disciples were asleep. Seriously, did they not feel the cloud close in or see the bright light behind their eyelids? I am amazed but not surprised. We are like that. We are asleep when God gives us an opportunity to let Him shine. We are bickering when people are starving for the Kingdom. We just don't get how close His kingdom is. When he finally gets rid of all of this sin we better get it.
God's Kingdom is at hand. Now.