It is better to do what you love for work, but if it is your day job that enables an unpaid passion, then your life is still sweeter. What is important is that you make time for your dreams, not whether or not you get paid for it.
Write. Start writing today. Start writing right now. Don’t write it right, just write it –and then make it right later. Give yourself the mental freedom to enjoy the process, because the process of writing is a long one. Be wary of “writing rules” and advice. Do it your way.