Ciao Gordon and welcome to our forum!
A drive from Rome north toward Tuscany takes you from 2 to 3 hours, depending on your destination in Tuscany.
Even if you arrange for your hotel or your farmhouse owners to arrive that late at night, I would consider the need to make a certain time a stressful way to start off your vacation!! So I would suggest spending the first night in Rome and then drive up the following morning.
Walking can be done everywhere in Tuscany - how much are you planning on doing and do you have a preference for difficulty? A perfect place for photography and walking would be both Chianti and the area of the Valdorcia, between San Quirico and Pienza and Montepulciano. If you stay closer to Siena, you'd be a bit more central to do both walks and day trips out to most everywhere during your week. Take a look at the places found in Chianti to get you started!