Attempted the Nordmarka Rundt:
http://www.nordmarkarundt.com/, a 100 km informal ski race through the massive (300 km+) groomed trail system in the forest north of Oslo. No course markings, so like Ski-O but easier because there were trail signs/lots of other racers to follow. Stayed on the race course until about 70 km, where my legs and shoulders felt done, so took a detour to a ski hut, where I thought that one of my relatives in Oslo could pick me up. I didn't know until I called him that the forest roads are off limits to most people, so I had to ski another 20 km back to the start, over a huge hill because of poor route choice. In retrospect it was good that I had to ski back so I could see the limits of my ski endurance (although it is very low right now because I only skied 5 times this winter before the race).
With the 70 km on course, a wrong turn costing me an extra 2+ km (unfortunately on a big hill), and 15 km + after I left the course, I'm guessing I went about 90 km. I started the race at 8 am and made it back to the parking lot at 7 pm, but stopped a lot to eat, change clothes, look at the map, and bellow at the forest in pain.
The race organizers leave it to you to choose classic or skate, and everyone except me and two others chose classic. After trying the race, I see why - the tracks were so narrow for 20 km+ of the 70 km I spent on the course, that I couldn't skate and had to double pole uphill. To make things worse, some of the tracks were soft enough at the edges that my poles would punch through on every double pole. I was in a world of hurt from about 55 to 65 km because of this. I think I could do the whole 100 km on classic skis, IF I skied the winter before it in preparation. Haven't done any classic skiing this year because of Klister conditions everywhere I go.