Writing a novel is not a spare-time endeavor, as we traditionally think. You know the old cliche where the newspaper man is hammering out the Great American Novel in the few hours a week he is both not working and sober? Well, at that rate he'd finish his tome in about three hundred years. Of course, there are those few writers, like Jack Kerouac, who load up on stimulants and crank out a novel in one sitting, but most writers have to think something before they write it.
For me, writing is a process of translating what's in my head into written word on the page. Imagine, for example, watching a movie and simultaneously transcribing it into the most interesting prose possible. That's what it's like for me. While I'm creating a novel it exists in my mind as mostly visual information, with a few snippets of memorable dialog. The visual information must be described and the dialog must be fleshed out into a natural seeming conversation between characters. Once the first draft is finished, the real writing begins, which is rewriting and rewriting and rewriting. It's a lot like polishing stones. By the time one of my novels reaches Amazon, I have read it, start to finish, fifty times, at least.
As for the spare time business, I do grab every spare moment to get some work done, but on the days that I set aside to write, I adhere to a strict schedule. I get up and have some coffee and try to wake up a little. At 8:30 am, I sit down at my computer and I go to work. I allow myself a few breaks during which I do laundry, load the dishwasher or pay some bills--then it's back to work. At the end of my ten hour day, I hope to have ten pages written, but I can't gripe if I have at least six.
While writing does require some self-discipline, it is something I love doing. Most of my self-discipline is required in not neglecting everything else in order to write all the time.