Ho hum.
I really think this has been answered above, but let us have another go.
Start by looking at the movement allowed in the terrain you are in. If that movement takes you into terrain that will slow you, you then look at the movement distance for that terrain; if you have already exceeded that by getting to the terrain you stop at the edge of the terrain, otherwise you keep moving until you reach the limit of the movement distance for the terrain you are entering.
So, an example using your skirmish situation.
If a Loose Cavalry UG is skirmishing and starts in Good Going it has a skirmish move of 3 BW (5BW - 2BW); we will assume for this that it does not roll a VMD that changes this. Let us say there is a piece of Rough Going 2 BW behind it in the direction of the skirmish move. The Cv usually has a move of 3 BW in such terrain, therefore, in a skirmish this would be 1 BW (again, assuming no VMD alteration). So you move the UG backwards; when it meets the RGo it has moved 2 BW which is greater than the movement it is allowed in a skirmish move in RGo (1 BW as above) and, therefore, it stops at the edge of the terrain.
If instead of a Skirmish it was doing a Run Away then the move distance in the RGo would be 3 BW, therefore, when it meets the RGo after moving 2 BW it still has 1 BW movement allowed and so could move that for a total Run Away of 3 BW.