I did this ride last November by myself. I went from Key Largo (the Marriott Hotel) to Key West in about 4.5 hours riding time. I stopped once at the 50 mile mark (water) & once at the 82 mile mark (due to a huge t-storm). It is a fast ride, mainly cause it is so flat. I don't remember exactly, but I averaged either 21 ro 22 mph on the ride. It turned out to be 105 miles to the most southern point bouy.
The roughest roads are from Key Largo to Taveneir. After that it gets a lot smoother, but you go from two lanes to one. There are wide shoulders, but depending on when you go plan on heavy traffic on a road that is 45 mph. I left as soon as the sun was up and got halfway there before the traffic got too bad. Please be prepared for locals who are jerks. I probably had 3 different vechiles yell at me during the trip about getting on the bike path. But the bike path is more for the tourist than for a road bike.
If you have any other questions, let me know.