continued from here. I've had a writer's block on moving forward. Like a knot in a muscle, you just have to rub it out.
The first 13 verses of Mark set the stage for Jesus' ministry. We meet John the Baptist, and Jesus is baptized by John. Jesus is driven to the desert where he is tempted in preparation for his mission.
Then 1:14-15 arguably give us the key verses of Mark. This begins the first half of the gospel in earnest (1:14-8:30). What did Jesus preach? He preached that the kingdom of God was arriving. The climax of this section is arguably in 8:27-30 where Peter acknowledges for the first time that Jesus himself is the Christ, the earthly king of the kingdom. 
So we have the announcement of the kingdom at the beginning of this section, but the disciples do not yet fully know Jesus' role in that kingdom. He is simply its chief proponent, now that John the Baptist has been arrested. But by the end of the section they acknowledge that he is in fact the Christ of that kingdom.
Jesus does several different kinds of things in the intervening verses. He recruits and trains followers. In Mark, recruiting Peter, Andrew, James, and John is the first thing he does after he starts preaching the kingdom (1:16-20).  Later he will call a tax collector named Levi (2:13-17).  Still later Jesus' followers will coalesce into 12 key individuals (3:13-19), probably symbolizing the restoration of the 12 tribes of Israel. Then Jesus sends them out to preach just as he has (6:6-13).
These 12 only scratch the surface of those who followed Jesus in Galilee. The story of Jesus feeding 5000 people one day alone points to a much larger group of Jesus-followers. The early church understood these twelve to be unique in a way that none since have been or could be (e.g., Acts 1:25; 1 Cor. 15:5). And while Paul understood himself and others to be apostles as well, witnesses to the resurrection (1 Cor. 9:1), he also likely understood himself to be the last of this group (1 Cor. 15:8).
Nevertheless, the Great Commission tells us that making disciples of Jesus didn't stop here. Jesus sends his disciples to go make other disciples. In Mark 6, Jesus sends out his disciples to assist in throwing demons out of power and in preparing people for the arrival of God's kingdom. In Matthew 28, his core followers are to teach his commandments to all the nations and to baptize them.
In recent times, we have tended to limit the Great Commission to convincing others to become Christians, but there is nothing so narrow in Matthew 28. Becoming a follower of Jesus in Matthew is more than getting baptized. It is even more than a process of learning Jesus' teachings. Becoming a follower of Jesus in Matthew means taking on Jesus' yoke (Matt. 11:28-30). It involves being willing to be persecuted, even crucified because of following Jesus (Matt. 10:38).
A second thing that Jesus did was cast out demons. When Jesus did so, he was doing something much bigger than just helping one person out, although Jesus' exorcist ministry was a clear example of his compassion toward others. Rather, many Jews at the time believed that the earthly realm had come to be in the control of demonic forces since the earliest days of humanity. Therefore, when Jesus cast out demons, he was reclaiming the earth for God--the rule of God was returning to the land once more (cf. Luke 11:20). And he sent his own followers out to participate in this sign of God's approaching kingdom.
Certainly Jesus heals many people in the first half of Mark, perhaps the most obvious example of his compassion. He heals Peter's mother-in-law (Mark 1:29-31).  He heals a leper (1:40-45) and a paralytic (2:1-12). He heals a man with a disabled hand (3:1-6) and someone who cannot speak or hear (7:32-37). He heals a woman with a hemorrhage (5:35-43) and even raises a girl from the dead (5:25-34).
Some of the miracles he did transcended human healing. On one occasion he multiples food to feed 5000 (6:33-44), on another 4000 (8:1-9). He calms the sea (4:35-41) and walks on water (6:47-53). These events speak of a power from God that we would call supernatural today, transcending the laws of nature.
Jesus prayed. Mark gives us only two verses on Jesus' temptation in the desert (1:12-13), but tells us elsewhere that Jesus sought on deserted places to pray (1:35). He would increasingly find it difficult to get alone for such things. If it wasn't the crowds seeking him out, then it was his followers seeking him out for the crowds. We can presume that this time alone with God not only recharged Jesus but also clarified for him his sense of God's will.
It is easy for us to forget that Jesus fully participated in humanity...
 In Mark's language, the kingdom of God is the kingdom of God, and thus God the Father remains the ultimate king of the kingdom.
See chapter 5 of The Essential Jesus for more about the followers of Jesus.
 Our impulse to harmonize makes us want to equate Levi with the disciple Matthew. After all, in Matthew's version of this story, the name of this tax-collector is named Matthew. At the same time, there are some questions about this identification. So a disciple Matthew is mentioned in Mark 3:18. If it was the same person as Levi, why wasn't he called Levi there as well. Mark 2:14 says that Levi's father was Alphaeus, but Mark 3:18 identifies a James as the son of Alphaeus. None of these observations preclude the possibility that Levi is another name for Matthew.
 It is important for some Christian traditions that the apostles not marry, but the most obvious reading of the New Testament is that almost all the apostles had wives. Cf. 1 Cor. 9:5.