The Circuit of Cornwall, also known as Kylgh Kernow, is a complete loop of Cornwall that can be accomplished by following the South West Coast Path and the Tamara Valley Coast to Coast route. You can start anywhere on the route, complete the entire Cornish section of the South West Coast Path from Marsland Mouth in the north to Plymouth in the south, and then link back to your starting point via the newly built Tamara Valley Coast to Coast route, which follows the Tamar river from its mouth in Plymouth to its source near the North Coast of Cornwall.
Although it's a challenging route, the experience of completing it is extremely rewarding.
