The text of today is about the successful return of the disciples from the mission to which they were sent by Jesus, and forms a sandwich construction with their sending (6,7-13). The verses that come in between are about the death of John the Baptist. Through this structure, Mark wants to indicate that the fate of John the Baptist will also be the fate of Jesus and of his disciples. Nevertheless, the disciples are to continue to engage in mission, confident in the knowledge that their Lord and Master will see them through.
Success and failure is often measured in terms of results. However, what is even more important is the amount of effort that each one of us puts into what we do. If at any given moment we can say that we have done our best and all that is required of us, then we do not need to be too bothered about the outcome.