I don't think the author of Mark accidentally left off a description of Jesus walking on Earth after the resurrection. I don't think that any more than I think the author of Mark accidentally left off a virgin birth story. I think the point of Mark is this: Jesus was right, Jesus won, you should follow Jesus. Don't follow the violent revolutionaries, don't follow the organized church, and don't follow Rome.
This reading compells you to ask what does it mean to follow Jesus? What does it mean to take his path over the path of the organized church, the secular government, and the violent revolutionaries? Mark does not have a Sermon on the Mount or on the Plain. (Neat Chart Comparison here.) Mark then is talking about Jesus' work with the crowds. Healing and teaching and caring for the poor and challenging the authorities.
Mark was Dad's favorite Gospel. Knowing Dad's less than academic approach to the world, I wondered if it was because it was short. As I learned more, I thought perhaps it was because Dad was so opposed to the liturgical, he was a bare bones Disciple and Mark is a bare bones Gospel. As I read it today, it is crystal clear. Dad brought refugees to this country; he started food banks; he started discussion groups to heal racial tensions. Oh yeah, and he loved to question authority.