I'm having a similar problem right now, I'll try not to bore you with the details...
I've started work on a Short film, but had all these ideas and thought it could expand into a feature. After one day, it was 5 pages - and I was really disheartened, as I wasn't sure where else I could take it - looking at that magic number of 100.
What I do is watch a film. Any film, preferably in a similar genre to yours. Pay attention, and see how much various parts have to do with the overall storyline/3 act structure. You'll see that most of it relates immediately, but you may have to make your own connections to the main plot. Base a character loosely on yourself, imagine what you'd do and where you'd go faced with whatever scenario the character is. Also, brainstorm side plots that could interweave with the original.
If all else fails, direct it in your head and think about what camera shots would look good, and how they could be used - then write around that.