Economy Workout
This workout trains your muscles to expect you to want to go really fast at times. It uses short bouts of walking as fast as you can for 30 seconds, slow for 2 minutes, repeat 8-12 times. It should only be done once per week.

Warm up for 10-20 minutes at an easy pace, stretch and do flexibility exercises.
Take off at a moderate pace for 2 minutes.
After 2 minutes, speed up to walk as fast as you can for 200 meters/yards.
Slow to a moderate pace for 200 meters/yards (about 2 city blocks)
Speed up again for 200 meters as fast as you can.
Repeat this 8-12 times.
Finish with 10-20 minutes of easy walking, followed by stretching.
Threshold Workouts

Threshold workouts bring your body up to the anaerobic threshold at 80-90% of your maximum heart rate. These get you and your body used to high performance. You should do 2 threshold workouts per week.

Threshold Workout #1 - Speed

10 minute warm up at easy walking pace, followed by stretching and flexibility exercises.
Walk fast for 8 minutes or 1 kilometer at 85-92% of your max heart rate.
Then slow down to an easy pace for 2 minutes.
Repeat this for 3-4 repetitions.
Cool down for 10 minutes at an easy pace.
The threshold pace is strenuous, but one you could maintain throughout a 10 kilometer/6 mile race. You will be breathing very hard and able to speak only in short phrases.
Threshold Workout #2: Steady State or Tempo

Warm up for 10 minutes at an easy pace, followed by stretching and flexibility exercises.
Walk 20-30 minutes at 85% of your max heart rate.
You will be breathing very hard and able to speak only in short phrases.
Cool down with 10 minutes easy pace.

source:walking.about.com