It's unavoidable. Every book needs editing, whether you hire a professional or do it yourself, somebody has to attack that manuscript and get it into shape.
Some things are easy, like overused words. You run one or two chapters throuh Pro Writing Aid and use the results to mount a seek and destroy operation through the entire manuscript. Hunt down all the vague words, all those pesky -ly words, and force every one of them to justify their right to be in your book.
It's easy, really, but the thing about doing this is that you're forced to make changes, which can snowball from altering one sentence to an entire paragraph. That BS is time consuming. It messes with your mind too because you have to switch between culling and creating.
You've written that Character A said something quickly, now do you leave it or change it?
In the strictest sense, you should change it, because OMG! An adverb! Kill! But let's be reasonable here, unless you've been lazy and they're everywhere, the dreaded 'ly' isn't all that bad. If Character A is quickly mentioning the weather to change the subject, meh, you can probably leave it half the time. If A is declaring his/her intention to join the dark side, you should change it to something more effective.
Editing is all about trimming and polishing. That doesn't always mean the shortest way to say something is the best way, but it's always worth considering.