30-30s, cleverly named, involve 30 seconds of going hard alternated with 30 seconds of going easy, repeated between 12 and 28 times. I personally find this workout can be fit into any stage of training (pre-competitive or competitive) by simply adjusting the number of repeats you do.
If you are looking to raise your general fitness (pre-competitive)you are better off to do more repeats, in the 22-28 range. The idea is for your heartrate and breathing to mostly recover between the first few and gradually recover less as you go on. (pacing tips below!) By the last few your breathing and heartrate should not be recovering very much at all.
30-30's also make for very good "sharpening" workouts during race season. It is an ideal way to practice going fast and work on speed technique without producing too much lactate and fatigue. By decreasing the number of repeats to between 12 and 18 it makes for a great engine revving session.
This is a seriously fun workout but can be deceiving. I find when people are first introduced to the idea of intervals in this format they have one of two polar reactions:
1. You do it HOW many times...?! yikes!
2. What? That sounds too easy to be effective..
The key to success here is all in the pacing. If you start too hard, you will find yourself thinking (and feeling) reaction #1. It is essential to be conservative for the first few, just because they are short doesn't mean you should go all out, you'll feel it by #8, I promise. As you go on, you can gradually push harder, as you might in a race but the idea here isn't to kill yourself, it is to ski at a consistent pace and stay in high zone3/low zone4 while maintaining or increasing speed.
The rest phase pacing is equally important. If you slow down to a crawl (those who give reaction #2 are prone to doing this) you will lose the benefit of the high heart rate without significant energy output. However, you do need to slow down enough in between so that you can maintain your pace on the work phase.
The best way to find your ideal pacing is by experimenting, just as learning to pace a new race distance, it might take a couple tries to get it just right!
Have fun with this and if you have questions, just stop me on the trails and ask,