I am curious, did this fast idle just start one day or did it slowly increase over a period of time.
There are multiple reasons why this might occur, air leaks, vacuum leaks, bad idle control valve connection under your air cleaner (this is what I am guessing). Remove your air cleaner, taking careful note of all the connections to it. Underneath you will see a round cylinder looking object with an electrical connector and a 1" tube connecting to it. Are the tubes hard and brittle? Have they slipped off?
Additionally you may want to check this thread: Fast Idle
Keep us posted,