For something that exists in the real world that we expect to come up regularly in the game, we’d want someone to do the research and summarize the results with, for example, a list of typical marching rates over various kinds of terrain. That’s the kind of real-world information you’d expect a wargame umpire to use in a map-based simulation.
In a hex-based wargame, you’d simply move your allowable number of spaces, with rough terrain costing double, etc., and there’d be zero room for improvising, because nothing outside the explicit rules is even an option.
Almost everything that happens in an RPG is outside the scope of the rules, and that works fine. Other things don’t need rules per se, but they have a right answer you;d like to be able to look up easily. How much does a warhorse weigh? How thick is a guard tower wall? Some things benefit greatly from rules, but not everything.