A speed workout is one of the best ways to kick your running program into high gear, but it doesn't have to be anything fancy. The key to an effective speed workout is speed. You can definitely get that doing repeats on the track, but you can also get in an effective speed workout while on the trail, running at the park or at home or in the gym on the treadmill.
Basically all you need is a series of slow and fast intervals. These intervals can be based on time or distance. For example you can run at a moderate pace for 4 minutes and then run fast (just below your 5K pace) for 1 minute. Or if you'd prefer to base the intervals on distance, try running a quarter mile at a moderate pace and then run a quarter mile at a fast pace. The key is to repeat this series of slow/fast intervals for at least 30 minutes.
Your workout might look like the following:
1. Begin with a 5-minute warm-up (a brisk walk or easy jog)
2. Select one of the following interval combinations below. Rotate between the slow/fast intervals for 30 minutes.
4 mins @ a moderate pace / 1 min @ a fast pace
3 mins @ a moderate pace / 2 mins @ a fast pace
1/4 mile @ a moderate pace / 1/4 mile @ a fast pace
1/2 mile @ a moderate pace / 1/2 mile @ a fast pace
3. Finish with a 5-minute cool-down (easy jog or walk)
Incorporate this workout once a week into your running routine and you'll see improved muscular endurance, improved VO2, improved speed, and overall better running efficiency.
For a recap of the workout, check out the following video clip from RunnerDude's Fitness.