LynnH, The first thing to do is to secure a permit from your county Public Work Dept to use the roads, if you can't get permits everything else you do will be waste. You'll need to meet with the permit office, and take details of the date, routes you want, location of congested spots like rest areas, and start-finish facilities and parking for 250 cars (participants and volunteers).
After permits are in hand, you can print up mailers, contact bike clubs in the state (they all will include you on the list of regional events in newsletters and websites). If you can, have someone make up a simple website with a printable registration form, send this website to bike clubs so that they can link to your website for their members. Also make up fliers to put in bike shops.