Here is an older article that has some good solid general training information based on scientific physiology studies.
A] really good background info
B] I think the point about intensity of intervals to increase your speed is right on…just a little over your current race pace. If you are looking at 50 km’s that will basically be at about your lactate threshold. If you are training for 5-10km races you need to figure out how fast you have been skiing—then increase that slightly for your speed training. Simultaneously you need to keep improving your efficiency on skis –your technique. That is a never ending process!!
Bill Meyer, Coach