Get an OS map, drive to Watlington or Princess Risborough and just explore. So many trails up/down/along the Chilters its pretty pointless trying to select a route for you.
Just ride about with the map/compass as an aid. If you dont like the trail you are on pick one of the many others. The hills wont kill you, some tracks will be muddy, others fine. The shape of the hills, prevailling wind, tree cover all conspire to create wet and dry parts.
Personally I would avoid the main signposted ones like the Ridgeway, this time of year lots of it will be pooey and muddy. I would go cheeky as well as off the beaten track, you will be on your own in a world of fun. Five mins away from the main towns / carparks its wonderfull.
Its great riding.