Every writer has his or her own process, and mine can seem pretty unique considering. I prefer to write a novel in 3-4 weeks, and then spend endless amounts of time editing. I went through a streak over the past few weeks where I was rereading old manuscripts that needed a lot of changes made. This ended up being more work that I'd anticipated. Not only was I editing for formatting, but content/grammar/spelling as well. I try to be thorough when going through a manuscript, and tend to edit in three phases.
First, I focus on one sentence, sometimes reading it a dozen times to make sure it makes sense. Next, I study an entire paragraph. Is what I'm trying to say translating on the page? Are there too many words, or too few? Finally, I look at a page or two as a whole, sometimes going back over them several times. Are my characters consistent? Is the story moving at an even pace? Do my characters have accurate motivations and reactions?
Yeah, I probably overthink it. But I'm a writer, and that's what I do. Time to get back to work...