Well that's my biggest beef with the Civilization concept: combat and unit management happens on the same timescale as scientific research and civilization development.
In other words, your unit will move on average two squares per turn, and sometimes this turn can mean 200 years, sometimes it means one year, but it's ridiculous to think that your unit has taken two hundred years to go from city A to city B. While the length of a turn is very well suited for city improvements and scientific research, it's really bad for warfare and unit management.
Ideally, there should be two different timescales, one is just like the regular Civ timescale, the other one would only happen in wartime, where the turns would suddenly be in weeks/days instead of years/centuries. That way your unit moving from A to B would actually take a time that's realistic to do it, and wars would be much more focused.
Once the war is over, the timescale switches back to the normal one.
Also, while in peace time, military units should be able to teleport pretty much everywhere on the map (adjusted to the age you're in: might take more, or be impossible, in medieval time to move to another continent or a really far away place)
Of course, constant war would be a challenge