Maps

This is Kusan and Musai:

They're divided by the Runias Mountains, which are huge and impassable; thus, anyone wanting to go from one country to the other either has to take a ship or go around the mountains at the northern and southern coasts.

Eristhenia is about a week's sail east from Saitome or Leana:

Merinha is a few days north and east of Eristhenia.