Reviving this thread just in case anyone else searches for the same information. I just got back from my trip to Tuscany. I ended up doing a 1 day bike tour through Florencetown. The cost was 80 euros, which included bike and helmet rental, english speaking guide, sag wagon, and lunch at a beautiful winery and farm. They provided transportation to and from the Florencetown offices in central Florence. I think it was 80 euros well spent.
The bikes were Bianchi mountain bikes. The pace was set by the group's slowest rider, but I was in no hurry. We were free to ride ahead and take pictures, as there were meeting spots along the way to re-group. The total length of the ride was 15-20 miles I guess.
If I were to go back on a vacation trip with more time, I'd probably look into renting a bike and creating my own tour. However, for someone like me visiting alone with little free time, the guided tour is a good way to get a taste of riding in the Tuscan countryside.