How to write a novel (like I know)

Robert McCrum in The Guardian usually has something interesting to say about writing and books but his latest blog left me wondering if I'm on the same planet as many other bibliophiles.
The piece asks whether writing classes are a waste of time. It's a valid enough question as we seem to be living through a boom period for them. Writing a novel, selling it for a million pounds and becoming pleasantly famous and well thought of as a result, has of late become a middle class equivalent of the National Lottery. Bored with being a lawyer/accountant/doctor? Then why not write a visceral account of your life in about 80,000 words and spend the rest of your days talking about it at book festivals (which you increasingly like to hang about in anyway since you stopped buying new music in 2003).
Creative writing classes have sprung up to feed this ambition. I can understand that. I've been writing books, as yet unpublished, for X years now. It's a frustrating business and I've often considered signing up for a course but usually I am put off by a number of factors. It's not the cost though. usually it is an uncertainty that a particular course could give me anything I couldn't get either under my own steam, or from my extremely experienced author wife. Course satisfaction also seems bound up with who your tutor is - another literary lottery.
But there is a role for creative writing classes however, and in this I think we can learn from American screen writing courses. Structure is what makes the difference between a well written novel and an unputdownable novel. Structure is all important in thrillers, comedies, sci-fi, and yes even literary novels (whatever they really are).
McCrum's post and the comments from the dozens of readers seem to confuse good writing with good prose. But prose, like coming up with good ideas, is something pretty much anyone can do. The trick is to string enough good prose together to make a novel.