It's a lot of work. A lot of work.
Think about it. I started writing with Jacky back in 2004. It started as an epic urban fantasy. I didn't know a spit about writing, like characterization, description, plot continuity, or anything else. I knew how to put words on paper--or screen, as the case may be--and that was about it. But I did a lot of it. No idea what the word count was, but I wrote 25 chapters of the very first original never revised/edited version of Jacky.
It sat for a while, and when I picked it up and read over it to see exactly what I'd done, I nearly gagged. It was horrible! There's a section where I used "water" at least three times in the same sentence.
But, there was something there. The characters, at least, worked. So I started over again. This time, I finished the first book. And wrote four and half other books. Full length, all of them, ranging from 80-150k. Then I went back again, compressed the first two books into one, and came out with DEALING WITH DEMONS. I was working on the second (originally the third) book of the series, when I had that horrible feeling something wasn't working right. DEALING WITH DEMONS worked. It was good. I wrote query letters, started to re-read it for the upteenth zillion time so I could put together an outline for synopsis writing, and realized it wasn't going to work.
Another book and a half of work, tossed out the window.
So I started yet again. I wrote 18k, tossed out 14k of it. Now I'm still not sure what to call this current incarnation of the first book in what I hope will be the Jacky series, and working away.
What did I learn in all of this?
It's really important to go back over what you've written. It's also important to let it sit for a little while before going back over it.
This doesn't have to apply to everyone, but for me, I need to revise as I go along. As soon as writing slows down, starts getting hard, and I have that niggling feeling something's not quite right, I hit the revisions. Usually I can find the problem, clear it up, clean it out, and get back to the writing. Other times...
Well, other times I end up throwing out over 3/4 of what I've written.
All's fair in the game of writing.