I almost asked the question "Can someone without formal training write a novel?", but the answer of course is yes. To get a better answer, I ask the following question:
How difficult will writing a novel be for someone without formal training?
I'm not talking about a best seller, but just something from start to finish that is readable. I've taken basic writing classes in college and high school, as well as some technical writing, but I've never formally taken any classes in creative writing. I've written the casual blog post for a personal page, but that is usually about current events or personal projects.
I know that as you write, you normally get better at it, but should I postpone starting a long story and focus on some extra classes/self-help books?