Cross-country skiing is a primarily aerobic sport. The best way to develop your aerobic system, and even your higher end fitness (V02 max and lactate threshold) is with easy to moderate (60 to 80% of max heart-rate) intensity distance (45min to 2hr) sessions. This type of training comprise about 80% of the training load, even for elite ski racers.
This being true, it is also the case that the training week should be built around one to three harder training sessions. A harder training session is either a short hard session or a long easy session.
For instance many programs are built around two interval sessions and one long (3hr) easy (heart rate around 70% of max) session. Your body adapts to a certain stress after 4 to 6 weeks and so if you don’t change that stress, doing what you have already been doing will only serve to maintain what you have built.
It can be helpful to look toward your racing season and plan backward. You should end up with a plan that builds toward the racing season. The basic idea is to build your aerobic base over the summer, work on more race like aerobic and anaerobic fitness in the fall and early winter, and race fast in the winter.
In the summer then you would consider doing mostly easy to moderate intensity workouts with one session a week of harder training, and some strength training. As the summer/fall/early winter goes on you extend the duration of the workouts gradually, making sure you get lots of rest so that you are getting stronger and feeling better rather then getting more and more tired as the summer goes on.
There is a lot of training material out there, but this is the basic idea: training breaks the body down, rest builds it back to a level higher than before training. Remember REST builds the body up.