"Sometimes that's a year, sometimes it's 18 months, where all I'm doing is taking notes. I'm reconstructing the story from the back to the front so that I know where the front is"

John Irving is referring to the procedure of writing a book. He is suggesting that it can take a long time to compose an unique, sometimes as much as a year or 18 months. Throughout this time, he is bearing in mind and rebuilding the story from the back to the front. This means that he is starting with the ending of the story and working his method back to the beginning. This permits him to have a better understanding of the story and how it need to be structured. By doing this, he can make sure that the beginning of the story is strong and engaging. This process of bearing in mind and rebuilding the story from the back to the front is an essential part of the composing procedure for John Irving.