Yes, if it really will take 120 hours, you have to plan for that at one level of your mind, the Rational Writing Brain. RWB allows you to estimate how long certain kinds of writing will take.
But to the Primitive Writing Brain, that 120 hours is an invitation not to start. PWB would say "120 hours? Okay, I'm out of here. No way am I sitting in that chair for 120 hours."
So RWB has to set to work and coax PWB with the Five Stages of Writing every day:
Denial: "Naw, it won't take 120 hours. Why, I'll bet that if you sit down today, you can get 5 pages done! Remember when you wrote X piece so fast? I'll bet it'll be just like that."
Anger: "Yes, it's lousy right now, and it's going to stay lousy unless you get to work and fix it. Get moving!"
Bargaining: "If you just write for the next 20 minutes/200 words, you can get out of the house for a while."
Cheerleading: "See, you're almost finished with this part! You really can do this."
Acceptance: "It's not so bad, after all, and this part is pretty good. You won't have to revise this again tomorrow."
See all the little white lies? Of course it will take 120 hours. Of course it has to be revised tomorrow, and tomorrow, and tomorrow, but if RWB said that to PWB, PWB would never let the writing alone for the day, let alone pick it up again the next day.
Writing's like making bread. No matter how diligent you are about kneading it, if you don't let it rise or rest at all, you'll never be able to do anything with it.
Want to hear the song? http://www.youtube.com/watch?v=qTRFPbEpun0