Interval training involves alternating "work" periods (sometimes also called "sprint" intervals) of higher intensity exercise with low intensity "recovery" periods. Each set of a work interval plus a recovery interval is commonly called a round, and your total workout is defined by a warmup, a desired number of rounds, followed by a cool-down.
High Intensity Interval training (HIIT) is a very demanding style of workout, and the phrase "high intensity" often scares many trainees, so do not be afraid to start out too easy and slowly increase the level of difficulty. When I was reading Turbulence Training, Craig Ballantyne had a great explanation for interval intensity. If on a scale of 0 – 10, 0 is not moving at all and 10 is running for your life .. then the ideal work interval is around an 8. A very important thing to remember though is that even 'running for your life' will be different speeds for different people.
Keep in mind that if you have not trained in a long time … or have never tried HIIT before, then even a work interval of 8 might be too much for you. For you, a good work interval might be a fast walk pace. It really depends on you and you should definitely not be afraid to start out too easy and progress to increased difficulty. You can always make it harder at the next workout, but if you go to hard before you are ready you could set yourself back with injuries or just get discouraged.
A Sample Beginniner's Workout
3-5 minutes easy walking
Walk at a leisurely pace. Just get the blood moving and get loosened up. Spend the last minute walking just a little bit faster to get you ready for your first interval.
30 seconds of fast Walking
Remember the 0-10 intensity scale? If you're just getting started with some walking intervals than shooting for a 6. This should be a fast / brisk walk. Imagine you are late for a big interview and really need to hurry. If you can not maintain this for 30 seconds, shoot for 20.
90 seconds of slow walking
Immediately drop your speed to whatever you need to catch your breath, but keep moving. A complete standstill is too taxing on the heart. As you recover you can bring the speed up again just a little. If you need more than 90 seconds in the beginning than go ahead and take it. Shoot for 6 rounds of this and then follow it up with another 3-5 minutes of easy walking as a cool down.
Do this 3 times per week, being sure to take a day off in between each workout. Once you get a handle on the 30:90 intervals, add an extra round in. When you can do 8 rounds of 30:90, then first focus on cutting down the amount of recovery time. Go to 30 seconds of fast walking followed by 75 seconds of slow recovery. Again drop down to 6 rounds. When you can handle 8 rounds again, cut another 15 seconds off of your recovery time and drop back to 6 rounds. This way you are not following a strict schedule that says you should be at this level doing X number of reps by day X, but are steadily progressing at your own rate. Be sure to log everything down in a journal so that you can see your progress.
When you are back to 8 rounds of 30:60 second intervals, then try upping the work intervals by 15 seconds while keeping the recovery period fixed at 60 seconds. Drop to 6 total rounds and work your way back to 8, then add 15 more seconds of work until your rounds are 60 seconds of brisk walking (likely to be almost a jog by now) and 60 seconds of slow recovery. By now, I promise that you will be transformed for a couch potato and ready (and hopefully energetic and eager) to begin pursuing more intents styles of training.