I walk, on average, a 15 minute mile and on flat terrain. I weight 150 lbs and I'm about 5' 7" with a medium build.
I'm trying to manage my overall health and want to ensure my exercise is balancing my food intake properly.
A reasonably accurate figure can be obtained by using a heart rate monitor (even one of the App's that use your phones camera will suffice). Assuming you're walking on the flat and not into gale, simply record a post mile heart rate, along with the time taken to estimate your VO2 figure per: Is it possible to measure calorie burn from heart rate alone?, and from the VO2 figure the calories.
You can also plug the heart rate into the Rockport Walking test formula, to obtain an estimated VO2max score.
See also: Walking Calorie Burn Calculator
Here is one calorie burn calculator (Shapesense.com) for walking.
I walk a lot and I have a walking tracker with calorie counting on the phone but I never use it. Calories burnt just don't tell me anything. I use the holes on my belt as a measure, seriously.
Walking alone is not that great to lose calories, unless you walk really a lot. A relatively easy way is to stop eating some types of food, for example all dairy or all sweats and such, and to eat whole-grain bread and vegetables, which prevents food craving a bit.