Vatnajökull National Park covers a huge part of eastern Iceland. Much of its interior section is a large glacier. There are wonderful hiking opportunities in the park, of various length and levels. In the Jökulsárgljúfur Section, mainly in the Ásbyrgi area (good Visitor Center), there are some hiking trails that are also great for running:
Run On Top of the Canyon: There are trails on top and below the canyon — one recommended route is the Eyjan hill (Trail A2 on the map, 4.5 km out and back ). Good surface for running, and flat. Great views into the canyon, and lovely wildflowers in season. It is also possible to run at the bottom of the canyon, on Trail A-4 (3.5 km one-way).
Run Between the Falls. Dettifoss is the most powerful waterfall in Europe. Selfoss, 1 km away, is another large waterfall and also beautiful. There is a pleasant and very runnable path between the two falls, making for a 2.4 km loop.