Push yourself to your limit with this 30-minute full-body HIIT workout for National Fitness Day. Gradually increase the heart rate and loosen up the body with a quick warm-up. Then work as hard as you can with a series of strength and cardio movements from press ups to burpees. You'll be doing 40 seconds of high-intensity work, before taking a 10-second rest and moving onto the next exercise. Run through the whole sequence twice before cooling down with some stretches.