Base training is so called because it is the base upon which later phases of training are built.
Aerobic endurance is the number one component of cross-country ski racing, and it is the component of ski racing which takes the most time to develop. It is the primary aim of the base training period.
Example: 2 hour rollerski or run split between level 1 and 2 or a 3 hour bike on hilly terrain split between level 1 and 2. Please note: about 80% of all training is endurance training. The rest is strength, intervals and races, etc.
General: Power and strength-endurance are built on max strength. General strength develops overall tendon and muscle strength necessary to support latter forms of training. General strength is the focus through the spring and summer.
Example: after building up to weight training for 5-6 weeks, include some ski specific high weight and low rep work.
Specific: Specific strength becomes more a focus later in the summer and into the fall once a solid base of general strength has been established.
Example: Endurance session using only double pole over gradual terrain.
Most intensity should be below the lactate threshold early in the summer. Anaerobic training such as speed is good, but hard aerobic and anaerobic intervals should be kept to a minimum early on.
Example: 2x10 minutes at 5 bpm below LT with 2 minutes rest between intervals. Start with 1-2 sessions a week.
Technique and speed
Speed training during the base period should not be done at a hard intensity (short bouts of speed with full recovery are recommended) and should be oriented toward using correct movements at race speeds - not at moving at an unrealistic pace.
Example: Incorporate 10-20 second bursts of speed into your endurance training. More details at http://www.skipost.com/training.htm