When you want to calculate the distance, you best use the GeoCoordinate class. It has a method called GetDistanceTo that will give you the distance from the used GeoCoordinate to a given other GeoCoordinate.
Details on MSDN here http://msdn.microsoft.com/en-us/library/system.device.location.geocoordinate.getdistanceto(v=vs.110).aspx