At over 5,000 years old, The Ridgeway Trail, may well be the oldest road in Britain. Drovers, traders, invaders and travellers of all sorts have all tramped this prehistoric path and today you can too. The trail meanders for 139 km (87 miles) over rolling, open downland to the west of the River Thames, and through secluded valleys and woods in The Chilterns to the east, never far from a cozy English village and lodging. This makes for great walking, whether for a multi-day itinerary or just for day walks. Along the way expect to encounter burial stones and barrows left by prehistoric man. You can read more about the Ridgeway Trail at the website.
Cicerone Guidebooks publishes a detailed guidebook for the walk called The Greater Ridgeway.