I did a tiny little tutorial on another site that sorta relates to this. Basically, duplicate your land shape and give the duplicate a stroke that's twice as big as you need. Half the stroke will be inside the land shape, and half outside. Lower this layer with the stroke below the original land shape then, with both selected, go to Object/Clip/Set. This will hide the stroke that extends beyond the edges. If you want a dashed stroke too, make another duplicate of the original shape, raise it to the top, and give it a stroke as normal.

I made a little pic previously, although it was more about giving blurred border, but the principal is the same. Hope it helps.

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