By Tracy Teare
Updated April 21, 2008

Part 1: Your weekday plan
Mark Fenton, author of the new book The Complete Guide to Walking, New and Revised: For Health, Weight Loss, and Fitness, designed these interval workouts to give you the most calorie-burn in the least amount of time—and to work around your busy schedule. Three days per week, choose one of the following 15-minute walks and complete it twice that day, either all at once or at two different times. If the workouts are too challenging at first, substitute two moderate 15-minute walks each day until you get more fit. (Just be sure to do 5 minutes of easy walking to warm up before and cool down after each walk.)

Walk 1. Alternate brisk and moderate walking between landmarks (speed up to the mailbox, slow down to the telephone pole, etc.) for 15 minutes.

Walk 2. Alternate moderate walking, brisk walking, and jogging for 40 seconds each; that equals 1 cycle. Do a total of 3 cycles, walk slowly for 3 minutes, then do 3 more cycles.

Walk 3. Find a long hill. Walk briskly uphill for 2 minutes, then walk back down at a moderate or easy pace. Repeat the process, walking uphill for 3 minutes, then for 2 minutes.

Part 2: Your weekend routine
Do one of these 50-minute walks on Saturday and one on Sunday. Both burn megacalories, so you'll still lose weight even if youve had to scrimp on one of your weekday walks. (Warm up and cool down for 5 minutes before and after each walk.)

Walk 1. Note your start location and walk at a moderate pace for 25 minutes. Then walk as fast as you can back to your start point, trying to get there in 23 minutes.

Walk 2. Walk briskly for 4 minutes, then slow to a moderate pace for 5 minutes. Follow with 8 minutes brisk and 5 minutes moderate, then 16 minutes brisk and 5 minutes moderate. Finish with 8 minutes at a brisk pace.

Why walk? It keeps weight off—76 percent of people tracked through the National Weight Control Registry who've lost an average of 66 pounds and kept it off for five-plus years walk regularly for exercise.