You do not need to know the ending of your story if you only have a beginning. And you do not need to know how your story starts if you already know how it will end. 

The point of creative writing is to write. Let the arc of the story appear in your process. Do not get hung up on starting at the beginning, writing the middle, and then the ending. 

If inspiration strikes on your day to write, and it happens to be a backstory of a secondary character, chase that stream and write.