Hi,
I just finished my 2nd year on a planning committee for a Mississauga based charity ride (
www.ridersagainsthunger.org).
Like has been said, start small, we had 36 bikes the first year and 70 the 2nd. This was a very manageable way to learn the process. The route needs to be set with safety in mind. Because you do not know who is going to attend your ride, you have to create a route that new riders would not find difficult or unsafe.
Have a well marked route and all riders should be given a map and directions of the route. If possible have road captains lead groups out, it is much safer. Contact a local riding club (Southern Cruisers, CMC), they are usually very experienced at riding with groups, they may offer to help you out.
Statr planning the route months in advance and ride the route the weekend before your ride to confirm no last minute changes due to construction.
Set up your website so that riders can pre-register.
Make sure you get local business's to sponsor the event. Set up sponsorship levels so everyone that provides the same dollar sponsorship gets the same exposure.
Have a strong organizing committee that is prepared to meet monthly starting in Jan/Feb to plan everything and keep everything on track.
Don't plan your ride on the day of any other large or local rides (Dad Ride, Bad Ride, etc)
The list goes on.
If you want some more info or need help shoot me an email and I will help out where I can.
Good luck, rides are a lot of work, but lots of fun.