(also found on http://neoendurance.blogspot.com)

This week I'll answer a question from Michele:
When you need extra time training (and you only have so much time) is it better to do longer workouts one or two times a week or five shorter workouts?
and Scott asked me my thoughts on High Intensity Interval Training:

Since these two topics are actually related, we'll talk about them both.
First, to address Michele's question, the answer I first would give would be "it depends." It depends on what your training goals are. If you are training to just improve fitness, then five shorter workouts might be the way to go. If I've got 5 hours a week to train, and only 5 hours, but I can mix the hours up any way I want, I might have a schedule like this:
Mon: 45min - intervals
Tues: 75min - moderate w/intervals
Wed: 30 min - easy
Thurs: 30min - intervals
Fri: off
Sat: 90min - moderate
Sun: 30min - easy

Here the workouts aren't too long (nothing over 90 minutes) and it mixes up intensity (intervals). It also includes some longer efforts for overall endurance.The intensity of the intervals might vary depending on goals and training block, but by doing intervals when you have a shorter amount of time to train, studies have shown that it can boost your fitness. Heck, cycling coaching legend Chris Carmichael has recently written a book call the "Time Crunched Cyclist."
If my goal was a long course race (say a half-ironman or marathon), I'd really want to make sure I was getting in longer workouts at least once a week, as I believe it is important to show your body the stresses of the longer day. So for that my 5 hour week might look like:
Mon: 30min easy
Tues: 60min - moderate w/intervals
Wed: off
Thurs: 45min - moderate
Fri: off
Sat: 30min w/intervals
Sun: 135min moderate

Here I've got several shorter workouts, one extra day off but I make sure I'm getting my long workout in.
You've noticed that I've put intervals into the workouts. Intervals are sessions of higher intensity work with a recovery period. A 45min workout with intervals might be 15min warm-up, 5x(2min hard, 2min easy), 10min cool down. If I don't have 45 minutes to train, what do I do? Some suggest High Intensity Interval Training (HIIT). The basic theory behind this is that if I go all out (really all out, not just "hard") for short periods of time with short recoveries, I will actually burn more calories and fat and improve my aerobic fitness. A typical HIIT session might be only 10 minutes long, or 20minutes total with the warm-up. I might do 30s all out, 30s easy 5 to 10 times as my main workout.

My opinion: If you are trying to maintain fitness and don't have a lot of time, or if you are trying to lose some weight, HIIT can be an efficient way to go. You are going all out - max effort for brief periods of time. But ideally we want to train the energy systems we will be racing at. It might work for a 5k, but if you are training for an endurance event longer than an hour, I don't think it is appropriate. You won't be working out long enough to stress your body like you would in a longer event. Now I'm sure there are ways you can work it into an endurance training plan (for example if you only had 20 minutes to train one day and you wanted to do something) but I don't believe HIIT should be the foundation of an endurance sport training plan.