I do that a lot -- especially when I'm working on an article or a new chapter in my book. Sometimes, I'll sit alone in a room with the door shut, eyes closed, my head in my hands as I think. I ponder the story I want to tell, how I want to tell it, and the best way to begin. To some it may seem as though I'm wasting time, but I can tell you that "wasted" time pays off in the long run. When I finally begin to write, it often cascades like water after I've spent the time thinking it through, or as Nervaudt said, it "becomes a toboggan and starts to slide." The writing flows freely and naturally if I've spent the pre-writing work and requires less editing and fewer rewrites.
While just doing the work of writing is important, don't neglect pre-writing. Sometimes, that's when the muse appears to show you the way.