Developing speed is ideal for time-pressed runners because getting faster requires short, targeted efforts."Quick bursts improve your ability to produce energy without using oxygen, while strengthening muscles - both of which help improve your speed

. If you’re running more than 16 miles, use the same formula, but only run the final three to four miles at marathon goal pace.Hills + 800sImprove form and fitnessCombining a hilly route with half-mile repeats elevates your heart rate, which boosts your

.You've got...An hour (or more)You should: Go longThe longer you're on your feet, the more calories you'll burn. Plus, long runs improve base fitness so you'll reap more from high-intensity workouts, says Shah.Here's howRun at conversational pace. Keep an easy

seconds of walking lunges.You've got...45 minutesYou should: Do an envelope run, body-weight resistance training and stretchingAdding flexibility work increases range of motion, which decreases injury risk and improves alignment, says Pierce.Here's how