Yes, there is no secret, if you want to perform in Nordic skiing, you will have to maintain your physical shape all year round. Cross-country skiing is a very complete sport and requires a lot of cardio, so endurance and weight training will be a good basis to arrive at the snow at your best! And overall, think about sports that promote coordination of movements and balance, such as yoga. Because to progress in cross-country skiing you will sometimes put yourself off balance.
Conversely, cross-country skiing is an alternative sport for runners who do not wish to run on snowy trails.
And if you are a fanatic you can also practice all year round thanks to the wheel skis!
In short, prepare yourself well because afterwards it's no longer a joke!