Most people hate the idea of committees. But if you have one, stuff gets done. And it gets done by more than just one person. You don't need a huge formal organization -- you need a few people who are as committed as you are to making this event happen.
The easiest way to find people to be on your planning committee is to ask around the Pagan community. Talk to everyone you know -- particularly those who know a lot of other folks, like shop owners, coven leaders, etc. -- and tell them what you've got in mind. If they're on board, schedule a meeting that everyone can attend, so your planning committee can start brainstorming for ideas. Six to ten dedicated volunteers can organize an event effectively. Be sure to schedule meetings in advance so people can work around other obligations.
To get the word out to members of the Pagan community you don't know personally, ask local shop owners if you can post a notice in their store about your organizational meeting. This should, obviously, include the date and time of the meeting, and say something simple, like "Would you like to see a Public Pagan Event? We need your help! Come share your ideas at [meeting time and place]."
When your planning committee holds its first meeting, you'll need to do your best to make sure no one person dominates the entire thing. Listen to everyone's ideas, and make sure anyone who has something to say gets a chance to contribute. For this first meeting, discuss possible names for your event, dates, and potential venues. Ideally, you should do this anywhere from three to six months prior to the event taking place. You can still make it work with a narrower window of opportunity, but it's a whole lot easier if you plan well in advance.