I wrote two novels twenty years ago or so when I first started writing to any great degree. If I remember, I belched them out quickly. I wrote them just to do it-- to see if I could do it. I'd heard it's good to practice first, as with anything.
Now that I'm again writing something that will be more than a novella, of actual novel length, I find a couple interesting things about the experience.
One is that it puts a strain on the brain. Especially when you start obsessing over details, holding the entire thing as a piece in your mind, every character and plotline, every view and feeling, including every word and trivial item of punctuation. Especially when you're up all night thinking about these things-- reading it over and over in your mind's eye to see how it reads. I can see a little, just a little, of the mental stress the novelist DFW put himself through in writing his massive tomes. He should've got out and freed his mind.
Another discovery is that I-- unaccustomed to editing my writing-- find myself rewriting a few of the sections again and again and again. They're not even particularly key sections, but more of bridge sections. I swear I've spent as much time writing two of them as the other 40-some sections combined, most of which came extremely easy.
This doesn't include two key chapters near the end, crucial scenes, which I have rough drafts of but have put off finishing because I find them somewhat depressing. One of them, more than somewhat. Getting into the minds of the characters at this point of the story gives me a headache. Things were so much easier when I was drinking heavily! I now have no defense, no mediation.