Now that you mentioned it, the idea of sending an army halfway across the globe without any penalties, always seemed too irrealistic to me, especially when the units are traversing inhospitable regions such as desert, jungle or tundra. There should be some limit to how far your units can go. This limit could depend on the level of development of your civilization or on the type of unit, for instance. Maybe there could be penalties in the unit maintenance or number of hit points. Or maybe there could be a chance of defection that increased as the unit got too far or if it was "pushed too hard". Back in Civ3 if you kept a unit in a jungle for too long, it would get "sick" and start to loose hit points.