Read: Acts 1:8 Jesus is spending his lost moments on Earth. How did He use His time? (Acts 1:1-8) He was with His disciples, He proved themselves that He is alive, thought them and gave them perspective about their future.
It is interesting that Jesus didn't spend His lost days to prove to the multitudes who He is and to preach the gospel on the whole Earth. Instead of preaching the gospel to multitudes, He invested all His lost 40 days only in to the disciples.
The key message in Jesus' last days are the promise of the Holy Spirit. Jesus doesn't give a command to be a witness, but give a promise for sending the Holy Spirit end empowering them to become a witness. The result of the coming of the Holy Spirit will be that we'll become witnesses of His to the end of the Earth. Jesus didn't formulate a strategy to fulfill the Great Commission, but He gave power for it. We don't lack plans or strategies or tool, but we lack power. When we walk in the power of the Holy Spirit than the Great Commission will be obeyed. There is a direct connection between being fulfilled with the Holy Spirit and the gospel getting to the end of the Earth. The reason the Great Commission is not fulfilled is not lack of money, lack of tools or lack of plans, but the lack of number of Christians walking in the power of the Holy Spirit.
Jesus didn't give a task when he said you'll be my witnesses, but he gave an identity. To be a witness is not an expectation, but a reality of our new identity. To become a witness is a sign, a sign of walking in the Holy Spirit.
The disciples want to know when the Kingdom of God will come to Israel. Jesus doesn't talk about timing and dates (matter fact he says that timing is not our business), but He talks about how the Kingdom of God will come. It will happen as the Holy Spirit transforms us and empowers us to become witnesses of His and the gospel will get to the end of the Earth. The most we can do to help fulfill the Great Commission is to help other Christians to understand and appropriate the power of the Holy Spirit. Only the Holy Spirit can transform someone into a witness. We can involve people into projects, we can give them assignments, but we can't give them power. We can teach technics or methods, but only the Holy Spirit can transform them into a witness.
The only way the Church can get to the end of the Earth with the gospel if there will be enough number of Christians who are walking in the power of the Holy Spirit. That's the greatest problem of the Universe is this: Christians not living out their full identity and appropriating the power God promised to them!