Of all the sports I've done, and competed in, XC Skiing is the hardest. It's incredibly taxing, more than cycling even. Your HR hovers in and out of O2
debt constantly, and recovery takes longer. You are also affected by temperature fluctuations which are hard to control. It's brutal. (Never tried rowing. My understanding is that long distance rowing is up there).
Northug is indeed an animal. Incredible ability to accelerate on skis when everyone else is at the limit. He's very driven, and has a hard headed attitude, like every race is to the death. He was a quadruple gold medalist at this year's World's. But he has been beaten.
Bjorn Daehlie was probably the best XC skier ever. He won over and over, everything. And he got started winning before the EPO era really took off. (Yes, XCS has had almost as many problems as cycling). Another guy very close on that top list is Thomas Alsgaard, who now happens to be Northug's teacher.
XC Skiers have frequently excelled at other sports. Vegard Ulvang was a powerhouse in other sports, and climbed Mt. Everest.
Andy Hampsten, Sean Kelly, other cyclists, Stefan Edberg, many athletes have used XC skiing for cross training in winter to not only stay fit, but arguably increase your fitness. But you do use your upper body much more in XC skiing than cycling.
Universal Sports covered the nordic World Cup tour this past winter. Many of the races were with no commentary other than what was on the event loudspeaker - in several languages (!). Even if you don't get Universal TV, they are free on the web, and very high quality webcasting. Very professional. Racing should start this December.