Writing the Great American Screenplay/Novel/Stageplay

I still don't really know what inspires me to write. I have always been better @ writing short stories, poetry, anything that doesn't require time outside of that initial inspiration. I get the idea, put it down, fix it and I am done. A larger project requires much more dedication and time. I have a great idea for a screenplay, and I have written about 20 pages. I still think it is brilliant (pardon my being so humble), but have not had the "light bulb" lately to sit back down and continue the story. I wish there were a way to just write. When I saw George Romero in person last year, his wife was also present. She talked about how George would write his pieces. He would sit in front of the typewriter/computer (I don't recall how he writes), and compose for 5 days straight. No food, drink, sleep, bathing. Just the act of writing. It was compelling. I guess to do that kind of crazy schedule, you really need a bursting head-ful of ideas. I have them, but with a normal day job, I don't have the luxury of sitting for days, to do nothing but write. Excuses, excuses. If I was really committed, I could find a way. To all those successful writers out there. How do you do it? What process do you take, what steps do you follow? How do you inspire yourselves to keep the ideas flowing, especially on something as long as a novel or screenplay? Do you actually write out an outline? Do you write like me, directly from the head; no planning, just writing down the information spilling from your brain? I have found that the best writing I have done on the screenplay, was an early Saturday morning many months ago. I woke up @ about 4am, and wrote 6 pages in one hour. I felt like that was quite an accomplishment. I was in the "zone" that morning. I want to know how to revisit that place. Are there certain things that others do to find that locale over and over? I would love to have a first draft of the script by the end of the year. Of course it is possible, but taking the next step; the timing is difficult. Anyone? Anyone?