THE DOG-WALKER: 1. Head for a park that is about a 15- to 20-minute walk away. Warm up with a five-minute stroll. Then do random intervals for the next 10 to 15 minutes, speeding up whenever you can, but don’t slow down for more than a minute at a time. 2. Ready for a little fetch? Instead of just standing still while your pup retrieves the ball, challenge yourself to complete as many reps of a strength exercise as you can before the little guy returns. Try jumping jacks, squats or lunges. On the way home, do another 10 minutes of unstructured intervals. Finish with a five-minute stroll to cool down.