The disciples were very confident that they had this whole message from Jesus under control.  After all, they told Him that they do believe.  When Jesus stated that each would be scattered and go his own way, I think He was trying to caution them that they were looking at this from the 30,000 foot level. He realized that they were yet to be tested by the world, that the difficulty of traveling on the surface would be too much for any of them unless they followed His example.  He reminded them that He had overcome the world, and implied to them that they would too, if they only followed Him.

But we know, from our own experience, that down here on the surface the road is rougher than it appears.  The hills are harder to climb and the valleys of our down times are deeper than they look from above.  We know when we are at 30,000 feet that Jesus will be there to help us, but how many times when we are on the surface do we forget to ask?