Usually, when I find myself riding one day and skiing cross-country the next, or even doing both in the same day, it happens at the end of ski season. This winter, though, I get to compare the activities side by side when neither is at its best.
A cyclist and a skier both flow through the landscape. When I have to choose an alternative to skiing I always hope I can ride. Running or hiking won't provide the same rhythm and glide.
Cycling and skiing each have their own rhythm. A skier works much harder on a climb, and steers more dynamically on a descent. Racers now talk in skiing terms when descending at speed, but rubber on a road does not act quite the same as a ski edge on snow. Cyclists can easily reach alpine ski speeds going down a hill, while Nordic skiers will get big thrills at lower speeds, on their light, fragile gear.
Using a fixed gear I keep the speed within a narrower range than I would if I used a bike that could coast at the full speed gravity would allow, or let me use bigger gears at will to sprint or draft trucks.
After skiing I feel warm for hours. Nordic skiing uses upper and lower body muscles, whereas cycling uses mostly the legs. I feel nicely cleansed after a good bike ride, but completely renewed from skiing.
Taking the fixed gear into the hills I can add some upper body effort to the workout, but it is hardly as complete as if I had been hammering on a set of ski poles for a couple of hours.
Because I am not getting to ski day after day, I have not developed all the upper body weight and strength I normally do. It's like starting the season over and over as conditions deteriorate and then return with each trivial snowfall. Alternating with brief periods of skiing I can snatch a few rides. These don't really build my cycling form, either. It's all way better than nothing, but it's just treading water. If I had serious athletic objectives it would really bug me. Instead I am just glad I can get out and do something.