I've been to Florida in January. And there are many Floridians on this board. The poker run I went to in Orlando started out around the mid-thirties. It warmed up to the mid-sixties. The Key's are a different story. They don't have the freeze warnings.
I guess it all depends on what your expectation of 'warm' is. I've been to BikeWeeks that started out at 80 degrees. And then never saw a seventy...
We had it narrowed down to Key West or Tucson Az. But then we found a great deal on an all inclusive in Cancun. I left it up to the wife and she decided she'd rather lay on a beach in the Caribbean than ride on a bike.
I'd say fly to New Zealand where it's summer in January and rent a bike. I know you said continental US but that's where I would go if could afford it.