For rural walking tours....take the 1hr 30 min bus ride to Oxford (bus leaves from near Victoria Station -called the Oxford Tube) - Oxford is a great place to walk around - lots of history, collleges and a river (the Thames) to walk by (from Christ Church College where Harry Potter was filmed in the dining hall)....also where Bill Clinton went to college (at University College on Main Street) - you can punt boats up the river or walk to a country pub (The Trout is a good walk and a great pub!....
Good luck,
Andy