I guess we might as well give this away.
Think about the 2D map of games like Total War, or Knights of Honor. There are roads on the map but your units really don`t have to take them.
With long-distance army movement over the world map it`ll work just the same way. You task your army on the world map, and it`ll take the fastest way there. If there is a sea in a way, your army will march to the shore, automatically board a transport ship and continue it`s travel across the water.
Although making your army move faster if road is available is definitely a fine idea! Something we`ll consider.
Also, there are roads in RTS-view mode, which really come in handy then moving through a forested region for a while - no forest on the road, units move faster.