The amount of calories you burn while walking really depends on how FAST you are walking, as well as the length. Easy to do on a treadmill, since it tells you how fast you are walking, and most of the time you will also get your calories burned (which I have found to be fairly accurate - within 10-20 calories of Spark's count).
If you are like me and walk a track, a pedometer comes in very handy! I just have a simple $20 one that counts my steps, and tells me how many calories I have burned. I will then manually put that amount in Spark under the Cardio section. A heart monitor would probably work better, since it's actually tracking your heart rate, but I don't have that much money. I know that there are calculations you can do, etc. to figure out how fast you were walking (how long did it take you to walk that mile?), but I'm too lazy to try to figure that out!
All in all, I personally would use the Cardio tracking on the Fitness page to figure out how many calories you are burning while walking!
SW: 149.2 pounds
MGW: 139 pounds by 11/5/10
CW: 146.2 pounds!!!! (HIT MY GOAL!!!!!)
GW: 122 pounds (by 1/1/11)
| current weight: 129.8