From an article in August Prevention
If it's an indoor treadmill, warm up walk at 3 to 3.5mph pace at a 2 degree incline. (My treadmill has an incline, but it doesn't show how much!) After a 2 minute warm-up, look at your watch and change the incline by the second hand. Every time it hits :00, raise incline 4 to 7 degrees. Every time your watch hits :20, lower to 2 degrees. After 18 minutes, cool down, easy pace at 1 degree incline. Finish at 20 minutes.
For outdoor walks, warm up at easy pace for 2 minutes. Then every time your watch hits :00, power walk with arms bent. Every time your watch hits :20, walk at a moderate pace. At 18 minutes, cool down with an easy pace for 2 minutes.
The Workout at a glance
Weeks 1+2 Three or four times a week, do either the indoor or outdoor interval for a total of 20 minutes. Each routine burns about 150 calories
Weeks 3+4 Walk 7 days a week. Continue your interval workouts; on alternate days, do a 30- to 45-minute walk at a moderate speed.
Keep it up: To continue losing weight, challenge yourself: Speed up the pace of your fast intervals, raise your treadmill incline higher, or add another 10 minutes to your workout.
I guess a study found that people who exercised for 20 minutes alternating between a high intensity for 8 seconds and a low intensity for 12 seconds LOST 3 TIMES MORE FAT than those who worked out at a constant pace for 40 minutes. I'm going to try this out.