Writer's Fallout Shelter

I am beginning to see a rather fatalistic trend in my writing. I do very well writing short stories, I excel in flash fiction, but I'm losing steam in this journey toward 'epic novel'.

I have the opening clear in my mind, and individual scenes play like mini-movies in my mind--the characters have invaded my dreams, for God's sake! I even have the end neatly tied up, but I cannot seem to fill the spaces in between. How does one stretch it out, short of boring the reader with unnecessary detail? And for that matter, how much detail IS necessary?

I've never been one to make charts and graphs of my characters and their statistics, I don't even break my work into chapters because I'm never exactly sure where to break. Could this be the problem? Do I need to incorporate more *wince* structure in my writing? Seems rather obvious, doesn't it.... Yet, I've always feared structured writing. I'm afraid of getting caught up in the prep work, and lose sight of the big picture.

Any advice would really be appreciated!