Its up to you as a GM to do as much or little overland travel. Just change the scale and use a marker for the party. Drop back to battle map as and when. The scale you choose depends on how far they are going surely. If going by boat then it would be a lot of miles, if on foot for a day or two then a bit less. I always think of it this way - if you were doing this with a whiteboard in a room with people then what would you use ?