I'm coming late to this thread but maybe it's relevant because we're moving into snow season.
In my experience, some runners successfully transition to xc skiing and some just don't. I naively assumed that being a pretty good masters runner would translate to make me a good nordic skier—and I turned out to be mediocre, no matter how hard I worked.
What's different? Skiing requires a sense of balance and weight shifting very different from running. If you've done balance sports like skating, rollerblading, etc maybe this would come easier to you. Skiing also requires a lot more explosive strength and muscle power. In this respect, it's more like cycling, with more alteration between high power climbing/sprints and low-power gliding. MUCH more upper body and more strength is required in skiing.
Technique is big differentiator. You can be fantastically fit but if your ski technique sucks you aren't going anywhere. We knew a kid who was top 5 in state XC and track multiple times. He tried nordic skiing, flailed all over the place, and finished races behind kids who were 4 minutes than him in 5k running races. You can see a similar phenomenon every year by looking at state meet results for the two sports. Certain kids place near the top in one sport but way back in the pack in the other sport. A few kids excel in both, but that's very hard to do.
I can relate. I'm a typical skinny runner without much muscle mass. When I started doing nordic ski races, I'd get passed by fat old guys with bodies like sausages stuffed into lycra suits. Being "gravitationally challenged" isn't as much of a handicap in nordic skiing. The people with good technique and balance just glided away effortlessly. On the other hand, some people who are mediocre runners turn out to be good skiers.
Young people can pick up skiing easier than old guys like me. I know one coach who insists that athletes have to start by age 11 to have any hope of becoming top skiers (there are probably exceptions but there's more than a grain of truth in this observation). Just be forewarned that skiing isn't as simple as running. Running comes naturally to most of us. Skiing not so much.