This
Sprint Exercise Article describes an interval workout that saves you time when you workout. I LOVE 15-30 minute workouts. It's easy to motivate yourself for a 20-minute workout. Some of us find an hour too large a time demand - especially with jobs or children to tend. A great anaerobic workout for alternate days (use the sprints for aerobic days) is the 21-15-9 workout. Pick 3 exercises each week and do a circuit of 21 reps of each, then 15, then 9, and follow with stretches. Warm up with a carefully selected first exercise (deep squats without weight, for example) or a brisk walk. The whole workout takes under 20 minutes. Aim for hitting the large muscle groups - for example, squats, pushups, and crunches (weighted). The next week, substitute single limb exercises (20 21 reps each side!) like weighted lunges, single arm military press, and dumbell deadlifts (not single limb, but a nice back exercise).
