So sun and lapidus are in another time period than the rest on the main island. Jack juliet sawyer kate jin hurley.... they are on the main island and the plane, ben, sun and lapidus are on the hydra island (where the cages are) and so the time is different there. So that is why christian says you have a long journey ahead of you. And when lapidus was forced to crash land the plane he says "is that a runway?" and that is the thing sawyer and kate were being forced to work on when they were living in the cages.
Any ideas why sun? we know she wasnt supposed to come back so that is one reason... and how would they get back?