Here's the scenario
A unit ran away from a charge towards difficult terrain, if they rolled -1 bw move they didn't reach it, however normal run away did, thus reducing max move.
I am probably missing something but this created the odd situation where a 1 was better, because as a run away I can not choose to reduce the move hence having to try and enter the difficult terrain and hence reducing its max move, hence never reaching the terrain etc etc, a parodoxical situation. We played that the unit stopped at the terrain edge as it seemed more logical
While on question, what happens if the run away hits an enemy unit, can't seem to find that either,
The maximum movement distance for terrain only applies if you actually enter the terrain, so the correct move is to stop on reaching the edge of the terrain.
If you Run Away or Skirmish and meet an enemy UG you stop a gnat's whisker away. This should be in the errata. It was clarified in this forum. You cannot use the universal 1/2BW shift to avoid enemy UGs.
Richard