Assuming you have a "continent" that is made up of "nations", and the nations are closed paths. For any given nation, some of the border is coastline, some is not. If I understand what you are asking, you are wondering how to get an "inner glow" on only part of the nation's closed path.

Answer: you can't.

It turns out, though, that this isn't really what you are trying to do. You are trying to put an inner glow on the coastline.

So, make a coastline path:

Select all the nations on the coast.

Duplicate them in place.

Put them on a new layer (on top of the nation layer).

Lock everything but the new layer.

Take the scissors or knife tool and start cutting the duplicated nation objects apart, separating the "coastline" part of their path from the non-coastline part. Delete all the non-coastline parts.

Now start joining the coastline segments together until you have a single enclosed shape for the coastline of the entire continent.

Apply the inner glow to this new path.