Want to take a week or two mid-July and ride from NYC down the east coast. Doesn't have to be all the way to Florida, but would want to spend a few days on a beach somewhere before heading back.

Does anybody know of any groups that organize long rides like this?

There is the Longest Day ride in NJ. 205 miles in one day from High Point to Cape may. June 11th. http://www.cjbc.org/