The Pewsey Avon Trail is a relatively new walking route (created in 2010) using existing rights of way from Pewsey to Salisbury.
In Salisbury the trail connects to the existing 34-mile Avon Valley Path to Christchurch.
The trail is accompanied by a walking guide (Pewsey Avon Trail by Chris Cole) which you can normally find on Amazon or from the White Horse Bookshop in Marlborough.