Although fairly long, the drive isn't that bad at all, especially if you did it on a Sunday when the traffic is usually a bit lighter (so long as you don't drive down late afternoon / early evening). It's autostrada almost all the way and it doesn't get busy until you hit Napoli. It may look awful on a map but the stretch down to Rome (especially on a weekend or around lunchtime midweek) is quite easy and you miss Rome altogether as the road loops around to the north. In my experience, that part of the road is rarely that busy either, especially on a Sunday when there are less trucks. There are also some wonderful places to stop en route to break the journey. In fact, why not consider overnighting somewhere along the way if time permits. Good to hear of people staying in Sarteano. It makes a good base and the old town is very charming, very untouristy. I met up with some people I met on this board who stayed in Sarteano last summer and they loved it.