Excerpt from Wikipedia; The Landsker Borderlands Trail is a waymarked long distance footpath in the United Kingdom running through Pembrokeshire and Carmarthenshire in West Wales.

While it's mainly done on skis in winter and in a multi-day fashion, crossing the Hardangervidda mountain plateau is a popular challenge in Norway. None other than Amundsen himself attempted to do it in preparation of his polar expeditions.

The Jubilee Trail of Lake Constance Disctrict (Jubilaeumsweg Bodenseekreis): A Hiking Trail in Six Stages