I'm putting the final touches on the third draft to my novel, Spirit Quest. Although, I could argue that this is actually the fourth or fifth draft seeing as how I've rewritten some chapters.
The first draft is simply getting your ideas on paper, writing it all out. The second draft is taking a long sober look at what you have and finding the real story. The third draft is shaping and polishing that real story.
Working on an existing draft is different than writing. Sometimes all you have to do is cut a word or a sentence, re-write a sentence, or add a new one. Sometimes, it cutting a paragraph or a chapter. And sometimes, it juggling the chapter order to improve the story's pacing.
Don't underestimate the amount of time and work required. The first draft is simply that, a first step. There's a lot of work and a lot of it is thinking work. Thinking about your story and characters and their evolution throughout. Thinking about your ending and making sure you set it up properly. That's another aspect not to underestimate: know where you're going.
Knowing the ending is so key to your whole story. Even after your first draft you might not know where you're going, but you do need to work on that right away as your address your second draft.
It was a great accomplishment to complete my first draft. But, I know now that completing the third draft is a whole other feeling. And it feels great.