If you haven't planned your story before you began, once the opening is finished, you will need to know where you're going. Planning ahead is a good thing to do. Think about those big pieces such as the dark moment, the crisis scenes and use the middle to lay subtle plans for reaching them and also for making sure what you're writing will make suense when you reach those scenes. You'll need to be subtle when you plant the clues along the way. Pushing them in the reader's face will probably stop them from reading any more. There's also something that youcan keep as a trick or a twist to surprise the reader. In my current WIP, I didn't have the twist until I reached near to the last scene in the book and I realized I must have had this in mind all the time. When I realized this, I had to go back and clean up some of the more blatant hints and blur them somewhat. So doplan ahead, keep things subtle and if you discover a twist, keep it secret.