Altra Kielder Marathon

The Altra Kielder Marathon is a scenic trail marathon staged at Kielder Water & Forest Park in Northumberland, England, and promoted by its organizers as “Britain’s Most Beautiful Marathon.” Founded in 2010, the event has developed into a full weekend of running and multisport competition, with the marathon joined by a half marathon, 10K, gravel duathlon and junior races. The 2026 event is the 16th edition of the Kielder Marathon.

The marathon’s defining feature is its setting. The 26.2-mile route follows an almost entirely off-road circuit around Kielder Water, which the organizers describe as northern Europe’s largest man-made lake. Much of the course follows the Lakeside Way through Kielder Water & Forest Park, combining forest trails, waterside paths and extensive views across the reservoir and surrounding Northumberland countryside.