In a fast-paced, time-conscious world like we live in, you have to wonder why aerobic exercise like running and biking still remain much more popular than intensity training. The reason is likely that, for short periods of time, it hurts.
Intervals push you into your anaerobic zone, where your heart approaches it's maximum rate. You enter a state that is not dissimilar to being in the high altitude death zone climbers experience as they near the summit of Mt. Everest, where they experience a condition known as hypoxia, which is dangerous when endured for a long period (hours) of time.
Most of us don't venture into that zone for an extended period of time, but even if only for a matter of seconds (like when you sprint a quarter mile around a track), it's very uncomfortable as you struggle for breath and lactic acid starts to burn in your muscles, like a self-defense mechanism for you to slow down, whether you want to or not. Being mostly pleasure seekers, we don't go looking for discomfort.
But, more and more, the scientific evidence shows repeatedly going into the discomfort zone for a minute or two is good for your heart and lungs, burning calories, and metabolic efficiency. Interval training is not inherently any more dangerous than long distance running if you don't have serious heart disease (and even then it can be good for you - discuss it with your doctor). Here are some good examples from exercise.com.
The best way to approach interval training is to go slow and gradually build up both the intensity (a heart rate monitor is nice, but you don't need one - you know when you are running out of breath and your heart is pounding) and duration (one minute intervals are a good place to begin). Whether you are going to sprint quarter miles or pump an elliptical, make each repetition slightly harder than the first. On the last few, see if you can spend most of the time in the top end of your discomfort zone because that's where you derive the benefits.
As this TOJ has rhapsodized many times, in the case of running there are other benefits that often puts it at the top of my exercise choices because of the runners high (a real phenomenon) and joy of being outdoors. Plus endurance running, where you go five, ten or more miles, presents its own kinds of pain to conquer in the leg muscles, joints, and feet. And if you race, the discomfort is guaranteed.
Intensity training offers only indirect benefits in any specific sport. Whether road running, trail running, bike racing, or cross county skiing, you have to spend most of your time training the specific muscles used in that sport. There is absolutely no substitute because optimum performance relies on specificity training. Lance Armstrong is a great bike racer, but a mediocre marathoner. Michael Jordan was a great basketball player, but mediocre baseball player. Neither spent the time training in their second sports as they did in the ones they dominated.
A TOJ is interested in good health more than athletic goals. Intensity training is short, sweet, and good for you, and afterwards you can enjoy a beer.
March in Colorado. Tonight it will snow again, though as I write this it is raining. The days are growing longer and warmer, but winter won't leave yet, and it can snow into May.
Yesterday I ran up a trail early in the morning on eight inches of crusty snow, packed in a corduroy pattern by a snowmobile. It was 24 degrees F, perfect running weather. I was on the same trail where I had my coldest run at -3 degrees F on January 9. Today I lifted weights and did some jumps and calisthenics at home. It was a good, tiring workout, but I missed being outside.