Put the pen to the paper or your fingertips to the keyboard and let it flow. Why worry so much about structure with the first draft? You're not going to sell it.

Imo, and I'm way way way down there on the totem pole, that first draft is all about unleashing a wonderful burst of creative energy, seeing what you've got, where it's going. After too many hours to count, when you're ready... that second draft is when you start piecing together the pace of the work with proper structure.