Jeff,

We did this several years ago during the Spring Daytona Biketoberfest. It was a little chilli back then but warmed up once we reached the Miami area.
Very nice easy ride, once in Key West, we ate a great lunch at a converted gas station, we took a group photo on the Pier with a cruise ship docked in the background (I think we had 14 - 15 bikes).
Did the photo with mile marker 1 and the furthest Southern Point in the USA. Afterwards we headed back up around 3:00pm, we stopped near Homestead FL. where we spent the night, we ate at a great restaurant there as well.

Being in the fall, temperatures will be between 70 - 85 degrees. I'm putting this on my calendar, I plan on doing this ride