The book (Spark, link above) analyzes a handful of studies comparing the correlation between the type and duration of exercise, and its effects. In short: you

The book (Spark, link above) analyzes a handful of studies comparing the correlation between the type and duration of exercise, and its effects. In short: you won't see any long lasting benefits from a single 15min. The best results were achieved at 75-85% of your aerobic capacity, at least a couple of hours per week (total). You can also benefit from mixing simple aerobic exercises with others that require more coordination and fine movement of your body - martial arts, for example.