INSURANCE!!!!!!!!
If you are going to organize a ride you absolutely MUST have insurance. Sorry, but that's the way it is these days. Contact your local cycling association. You should be able to tap into their insurance provider.
Next, if you are expecting any quantity of riders ... enough to clog up the road a little bit in places, you have to get permission from the local police force. It becomes a public gathering or parade or something. You may have to pay a fee to be allowed to do that.
If these things are too daunting for you, have a look at my Links page:
http://www.machka.net/links.htm
On it you'll see various Randonneur and Audax organizations ... these all have long distance rides (from sun up to sun down, and longer). Check out the 24 hour events. Most 24 hour events have a 12 hour option. And definitely have a look at the UltraMarathon Cycling Association link.