Offa’s Dyke Challenge. Offa’s Dyke is a linear earthwork structure which roughly follows the border between England and Wales. Named after Offa, the Anglo Saxon King of Mercia between AD 757 and 796 who is believed to have ordered its construction. The Dyke in part can be measured as 20 meters wide by some 2.5 meters high. Running a total distance of 177 miles the route offers a great challenge to walkers and groups wishing to complete a lifetime bucket list challenge and is often completed by groups or individuals seeking to raise funds for charity.