The Active Northumberland Kielder Marathon route follows an almost entirely off-road course around northern Europe’s largest man-made lake, Kielder Water, in Northumberland. Britain’s Most Beautiful Marathon is the finale to a weekend of events, starting with the 10K & Gravel Duathlon on Saturday, and followed by the Kielder Half Marathon and Junior races on Sunday. It all adds up to an electric atmosphere and a fabulous weekend in this beautiful part of the country.