Just start writing and don't worry about it. Lots of good books are made in the edit, like carving a piece of stone. Be prepared to lose big chunks of stuff you've grown really attached to. "In writing, you must kill your darlings.” --William Faulkner Even if you only end up using a partial sentence of what you write, it's still good practice, and creating a world is as much about what you decide isn't fitting as much as what is. On a note about Alien and the Ripley character - it was written in the script as a male character. Which is interesting as that film is so often seen as a high point in three-dimensional heroic female movie characters.