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.
Monday: speed and spenst. Spenst = as per the spenst email. Start the workout with a warm up, spenst and then do speed. Speed = controlled efforts at higher than race pace of 15 to 30 seconds in ski terrain. Start with 5 to 10 sprints and build to around 20 taking 2 to 5 minutes rest between. Or you can simply build sprints and spenst into a distance session so that as you run or rollerski along you sprint or jump up hills as you come to them.
Tuesday: Easy distance session (1 to 2hrs at around 70% of max hr). Strength training = high repetitions (20 to 30) with lower weight. Weight should be such that you cannot do more than 3 sets of 20 to 30 (you finish the first set no problem, struggle at the end of the second set, and have a tough time getting 20 to 30 on the last set.) Focus on ski specific strength including a lot of stomach and back work.
Wednesday: Easy distance or off.
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