Now if you classed any as difficult terrain, then the rangers ability comes into play.

Terrain's effect on play is generally best determined by the players simply talking in advance of the game's start. We lay out terrain based on the scenario setup and then go thru the pieces and determine what we think. If any questions or disagreements come up, we throw a die.

So it might be appropriate to lay it out, say that all of it is normal, passable terrain except certain pieces or areas. Have those be difficult rocky and let the appropriate special rules apply.