Douglas Brooks, in his book Program Design for Personal Trainers, calls this a "spontaneous" interval, or "speed play." You're basically having fun -- deciding when to push the gas pedal and for how long, doing it kind of by instinct. It's a great way to start, and you can vary the "gas pedal" interval by time, intensity or frequency at your own pleasure. The recommended ratio of interval to recovery time is 1:3. So if you pushed the gas for 20 seconds, you'd spend a minute (3 times 20 seconds) in recovery mode before doing it again. Of course, in the very beginning, you may want to vary that and only introduce one or two intervals into the day's workout. That's fine. You get to choose how many "spontaneous intervals" you'd like to do in any given workout.

Next up in the intensity sweepstakes is a more structured pattern. For general aerobic conditioning, Brooks suggests that you use a 1:1 effort-to-recovery ratio. The "effort" interval should remain moderate, since you're going to do it for longer. Both effort interval and recovery interval should be 3-5 minutes. So, for example, if you're walking at a 2-3 level of difficulty, you do your effort interval for 3 minutes at 4-6 and then return to 2-3 for the same amount of time (3 minutes). Alternating these three-minute intervals of hard and easy gives you a lot of additional calorie burn and increases your aerobic conditioning. The number of cycles you do depends on your own goals, time and conditioning level.