Good: Running each repeat "hard," such as 4 x 3 minutes hard with 2 minutes easy
For beginners, these informal interval workouts are good for boosting fitness.Better: Running each repeat at 5K race pace
For intermediate runners, more formal interval workouts will prepare you for racing.
Great: Matching the speed of the hard efforts with the purpose of the workout
For advanced runners, it's important to know the purpose of the workout so you can train more specifically because endurance training and speed training require different workouts. The more specific a workout, the better. How do you know what speed to run each type of interval workout? The best interval training to enhance your cardiovascular endurance and your ability to transport oxygen to your muscles is 800- to 1,000-meter repeats (3- to 5-minute work periods) at 2K to 3K race pace. To increase speed, run 8 x 400 meters at 1,500-meter race pace with 2 minutes recovery jog or 5 x 400 meters at 1,000-meter race pace with 3 minutes recovery jog.
Long runs, which should comprise 25 to 30 percent of your weekly volume, deplete your muscles' store of carbohydrates (glycogen). This stimulates a greater storage of fuel, and increases your endurance because carbohydrates are muscles' preferred fuel. Long runs also improve your ability to transport and use oxygen in your muscles.
Good: 20 miles at an easy pace
For beginners, the most important part of long runs is the time on your feet.
Better: 10 miles at marathon pace
For intermediate runners, adding some quality to the long run improves your endurance.