This route traces for a good part one of the race routes of the Eroica: the Cento km di Val d'Arbia.
I did the route by MTB with my wife so very calmly dividing it over 2 days with departure from Monteroni d'Arbia and overnight stay in the area of Gaiole in Chianti (where you can find agritourisms and wonderful castles), however it is doable in a day. The route is 40% or so on asphalt road so a gravel bike is the ideal bike, but it can also be done with a road bike, touring bike, etc., as long as you don't underestimate the uphill tears that require good leg or agile ratios. Siena can also be a good starting point if you do the ride in the day.
The route is technically very easy, physically it is quite demanding because of the constant up and down, and in summer heat and dust. I recommend it in spring or fall. The views are stunning, those who love Tuscany will not be disappointed.