If you are creating the map using the method of extruding a polyline drawing, you can use an offset feature to reduce the outside by the desired amount. I'm barely literate in Adobe Illustrator, but can recommend Inkscape to perform the offset task. Fine details along the edges will be lost, but you wouldn't have much in the way of fine details on something 3D printed.

If you are working with existing 3D models, there is also an offset feature in OnShape and Fusion 360, but you're on your own with those. I suspect they work in the same manner as Inkscape, though. Of the two, OnShape will be the easier to learn that specific feature, as it is more user friendly, in my opinion.

Please reply with additional information regarding your source material.