[Fate]

I think BigJMoney had a good point, commanders will avoid bad terrain. In swampland this is hard, if not impossible, so that could be left. In a forested or mountainous terrain, however, they will often look for a clearing/valley to fight in.

I would therefore propose that forest/mountain survival units could take the most direct route (through the forest/over the peak) whereas those without the necessary survival would have to go the long way round. Would it be too hard/unbalanced for those without survival to start with 10 - 20 fatigue (to represent the extra walking)?