The Ridgeway isn't too bad at the moment, i'm just outside Wantage and its pretty smooth, but it can change to being like glue with a days rain.
If its dry i definetly think your stop over is possible. I've done Wantage to Goring and back again in just over 3 half hours when it was dry and managed to keep up quite a high average speed with a few downhills around wantage to keep it interesting.
Hope the weather holds as it is quite scenic around the white horse, and you do get some very good views from right at the top.