Much will be required of the person entrusted with much. Peter did not know what to make of Jesus’ saying on preparedness (he even calls it a parable!) He might have thought that as those who were already “members,” their place in the kingdom was assured. If anything, as companions of Jesus, they were the biggest “shareholders” in this project of the kingdom. Jesus must have read what was in Peter’s mind and proceeded to tell him: “Your being with me is precisely the reason why you should take this saying seriously.”
Peter and the other disciples’ place beside Jesus was more of a responsibility than a privilege. Jesus had revealed to them the secrets of the knowledge of the kingdom (cf. Mark 4:11). As companions of Jesus, they too shared in the ministry of advancing the kingdom. This they were to accomplish by remaining true to their call even as they proclaimed the kingdom in word and deed. Peter and the other disciples were to be on guard because by being Jesus’ companions, they had become responsible, not only for their own salvation, but also of those who would later be entrusted to their care.    
The preparation required of a disciple is the awareness of who he/she has been called to be. Like Jesus, a disciple lives not for himself but for the other (laying down his life for his friends). Jesus knew the temptation that would come the positions in which Peter and the others would find themselves (the temptation to lord it over others). Yielding to such temptation would go against everything that their “project” was about and would make them lose their identity as servant leaders.