I have just finished the first draft of my second book, such a great feeling. I was happy with the way it turned out. When I started I had only planned it in fairly broad terms. It is incredible how ideas can pop into your head once you start writing.
I would encourage anybody to have a go even if you dont know exactly how the story will turn out. It is a lot of fun seeing the story and characters develop.
The golden rule is 'Decide how many words you are going to write each day (500 in my case) and stick to it.'