I identify four kinds of editing. The first is that with which everyone is familiar: the solemn, miserable grind of checking for spelling and grammatical mistakes, correcting the order of words, removing unwanted adverbs and adjectives, getting rid of colloquial phrases like "in fact" or "as far as I'm concerned' and so on - this last being something that clutters my first drafts like ten drunks in a back alley. It has to be done, but after years of doing it the dull process begins to be rather straightforward. I usually set aside a certain amount per day, depending on the length of the work and the approach of the deadline: about 3,000 words a day. That takes me about an hour and a half, or more depending on the amount of work needing to be done.
The second form of editing is continuity. This is a significant problem in a novel. It is important to remember when the hero hits the villain with his closed fist in chapter 12, that you put a high school ring on the hero's finger in chapter 3. It won't do to say early in the book that the character's father was a mailman, and then to have a stroke of inspiration late in the book where you reveal the character's father was a prison guard. Something has to be worked out, whether you pick one or the other, or give a reason for his father having once done both jobs.
Here another set of eyes on the volume can be a big help. You will tend to be wrapped up in the language and structure of the text, while someone who knows nothing about your novel will be more likely to see those continuity errors. Only, be sure it is someone who will squawk when something is wrong ... I've known too many readers who will simply assume there must be some reason for you making the mistake, when in fact you've just forgotten.
It's easy to take more than a year to write a novel, particularly if it is something you don't do a lot, and that increases the likelihood of continuity errors occurring. If it helps, write out a list of pertinent facts about the characters, without looking at the novel. Then you are more likely to write out the things that are actually important, rather than a reflective list of things you've already written. This will also help with the third kind of editing.
This would refer to the importance of getting rid of things that are not working. The reference may not turn everyone's crank, but I always liked the line Bill Murray had in the movie Tootsie, where he confesses that the solution to his play is to write the "necktie scene" without the necktie. This can be a difficult decision. We fall in love with characters, or sequences, or little in-jokes, and it turns out they just don't work in the story ... but we refuse to accept that, and we go on making the problem worse and worse, until finally we have our own personal Jar Jar Binks. It's easy to see why Lucas couldn't see past the disaster that character was; he was too close to the work.
I have been pushed to the point where I have removed up to three complete chapters from a work, recognizing that the book was going in the wrong direction. It can be very hard to throw out characters, or settings, or even certain themes when you realize the work is trying to do too many things, or that it is doing things the wrong way. But do it. Kill the character. Get rid of that section, no matter how beautiful it is with the girl and the high wire and the tutu that rips and sends her to her death. If it isn't working, it isn't working. You have to be prepared as a writer to stab your novel or story right in the heart if it has turned against you. Feeding it more time just makes it worse. The same is true of a composer, a painter, a sculptor or any artist. Cut out the bad and move on.
Remember, though, that once you have cut that out, you are going to create continuity errors with what's behind ... when you get rid of the priest from chapter 9, don't forget to remove the reference to him in chapter 14.
The fourth sort of editing is the worst. Yes, it hurts to kill chapters, and it can make it seem like the book is never going to be finished, but it is worse to finish a book and realize that it doesn't have enough. You've come up short and you haven't any idea how to fix it.
My first draft of Act of God, I realized that my lead character, the narrator, was a ripe asshole. I had written him as a cook, and had included the sardonic, often mean attitude most of the cooks I worked with had. He was intended as a little-educated, tough, disbelieving sort of runt, and when I found I'd managed that acceptably, I realized he was impossible to warm up to. The reader wasn't going to get past chapter three listening to this guy. I needed something to make him more likeable.
My solution for the second draft was to give him a disability. He would have a crippled leg that would counteract his irascible nature and give him a little sympathy. The new feature had to be worked into every moment and every aspect of the story, and as a result I had to throw out passages and a full chapter, necessitating the rewrite that would ultimately change the climax of the story in favor of the character's physical flaw. In turn, this changed the character himself, so that he became less cold and mean and a little more funny, even compassionate in some instances. Overall, it heightened the work completely ... but it was a long, difficult effort to rework the whole book to fit that one highly significant change. Worth it, yes, but not much fun at the time.
There you have it. Any English graduate can manage the first and second kinds of editing, but it takes a writer to recognize when three and four are required, and it takes a writer to get dirty up to the elbows and do it. Don't hesitate, and don't put it off ... it only takes a great deal of thought, patience, innovation and a willingness to cut like a surgeon.