I am convinced, nordic skiing is by far the hardest physical activity that I know of. In no other sport have I felt like I was using my aerobic and anerobic system and weight training at the same time. Triathlons are hard for sure, but in triathlon you don't have to use every muscle in your body at the same time. When skiing I can feel every muscles from my feet to my shoulders are working. My arms, legs, and lungs burn alike. It's no wonder the best athletes in the world are nordic skiers. If you don't agree you are 1. crazy 2. haven't cross country skied, or 3. have the most developed cardiovascular system known to mankind.
Skating is so hard, after an hour of skating my g. maximus, g. medius, and hamstrings are just thrashed. Skiing hard for more than 10 minutes consecutively is quite possibly on of the most taxing things I have ever done. After and hour of skating I am fatigued, but satisfied knowing I "did something."
I am always impressed by the winter biathletes. They ski around the course at what I am guessing to be between 80 and 90% of their max heart rate and then have to stop quickly and shoot 5 targets with a racing heart. Winter biathlon is insane, it't insane to go from an aerobic activity (skiing) to a tecnhical skill (shooting) it's really an amazing sport to watch. I have had some luck being able to ski without aggravation to my illiotibial band so I plan to emphasize that over the winter months and hopefully kick the itb soon.
Last year I built up to 2 hours and 45 minutes of skiing in one session and I would like to hit the 3 hour mark this winter. I figured that would pretty much thrash me as much as the long brick workouts that I cannot complete currently.