There is no way you can measure it, but it is a fairly accurate way of measuring distance you have walked.
As for calories, these are burnt depending on precived effort. During a work out, calories burnt per unit distance start to fall at first then rise as faitgue starts. Calories per unit distance depend on your speed aswell. Again this falls at first and then as you pass your optimum speed (a fast walk i would imagine) it rises again.
There are many other circumstances that change the calories burnt, such as terrain, wind speed, gradient, temperature.
Then of course there is the fact that every one is diffrent, fitter people will burn less calories per unit distance under the same conditions as a less fit person.
The only way to accuratly measure calorie output, is with a heart rate monitor (unless theres some laboratory way i dont know of) at least certainly the best way avaliable to you. Even this, im sure is not perfect as your heart rate can be raised by other factors such as shock or exitment, and will be lower if you have over trained.