끝까지 완주합시다. If you are on a walk race or a walk marathon, you don't need to specify the act of walking since both parties know that they're engaged in walking. But if you are using "walk together" as a metaphor for living a life together... 계속 끝까지 함께 걸어요. Good luck!

계속해서 함께 걸어갑시다.