Correct way to calculate distance in meters?

// The calculation below is based upon a calculation found on StackOverflow:
// http://stackoverflow.com/a/11172685

// This code runs on an Azure WebService and I have found no way to use GeoCoordinate to access the GetDistanceTo method,
// which is why I've tried to solve it this way.

// Calculation starts here
var R = 6378.137; // Radius of Earth in KM
var dLat = (Marker.Lat - Lat) * Math.PI / 180;
var dLng = (Marker.Lng - Long) * Math.PI / 180;
var a = Math.Sin(dLat / 2) * Math.Sin(dLat / 2) +
        Math.Cos(Marker.Lat * Math.PI / 180) * Math.Cos(Lat * Math.PI / 180) *
        Math.Sin(dLng / 2) * Math.Sin(dLng / 2);
var c = 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
var d = R * c * 1000; // d = the distance in meters;
// Calculation ends here

if (d <= Distance) // if the calculated distance (d) is less or equal to Distance it should be included in the results.
{
   FinalMarkers.Add(Marker);
}