Basically, Marlena has always been a Sword & Planet protagonist, just no one ever wrote the story beats that come before "marry local royalty".
You need something exciting to happen with purely local elements before the Horde invasion. Maybe you want Keldor to be fighting Randor for the throne by the time Marlena arrives. Maybe Miro is still around and they have to fight the Enchantress. A side quest could imperil them in the Vine Jungle with Gygor and young Beast Man.