Nice hike, got lucky with good weather and great views. Lots of people on the mountain - there were about dozen on the peak when we got there. The normal route is mostly the 'S3' trail clockwise, with a short detour at the beginning to see the waterfall Svartifoss. It's all class 1 except for some class 2 when you depart 'S3' onto 'S4' to tag the summit. Total distance was 18k, or 11 miles. You can refill water at Svartifoss, then again from a brook at the northern-most point on the 'S3' trail, and that's it until you're near the end. Confirm those water sources with the information desk before you head out.