In a nutshell:
The first 6 km are rather tough, with more or less constant climbing. However, the last 4 km are mostly downhill towards the finish line.
Start: 290 m.a.s.
Highest part: 501 m.a.s. (Sandfell)
Elevation: 211 m
Longest climb: 82
Total climb: 245 m
You start from the ski house at Seljalandsdalur like all the other distances. The first kilometer is an easy loop around the start area, but the next kilometer is mostly a steep and difficult climb up to Háabrún, one of the most picturesque points of the course. The view from there over the fjord Skutulsfjörður is stunning and ideal to take a photo or two if you´re not too much in a hurry.
Following the climb up to Háabrún you come to a relatively flat section that stretches for just over a kilometer. It takes you under a ski lift and eventually to the crossroads where the 25 and 50 km skiers keep on going straight, but the 12,5 km skiers make a right hand U-turn to start another climb. Even though this one is not quite as steep as the first climb, it is still pretty tough, but the viewpoints on the way will make it all worthwhile. If the kick wax is working like it should, you will be able to use your long and beautiful stride all the way up. Once you are up on Mt. Sandfell, the terrain gets easier, although you are still gently working your way up to the highest point of the course. When you reach the “peak” you can see Miðfellsháls in the distance. This is the notorious last hill in the 25 and 50 km distances. Thankfully you are not going up there, but soon your 12,5 km course merges with the others as you start your 4 km descend down to the finish line.