Draw Route X Kilometers from Origin - google-maps-api-3

Question

Running/walking distance display.
User enters a location and a distance.
I can overlay a circle with a radius of the distance the user entered, with the user's location as the center point.
I can set four cardinal points (N, S, E, W) around the point of origin at the distance the user set and draw the routes to those points, such that point B is 100KM from point A, but the mapped route is, say, 145km along the road.
Is it possible to display a route along the road exactly 100km?
Edited to update progress.

Answer 1

Finally solved this and thought I'd share.
so, the user supplies a location and a distance; we'll say 100Km.
The code finds cardinal points 100Km N, S, E, W of the point of origin, then solves routes to each point. If solving for the route is successful, the result contains an array of points from the point of origin to the destination.
directionsService.route({
origin: start,
destination: end,
travelMode: google.maps.DirectionsTravelMode.DRIVING
}, function(result) {
renderDirections(result);
});
//don't use the google api DirectionsRender() to draw the route.
//instead - result holds an array of lat/long points that make up the entire route. Lets say it's latlong[123]
//iterate through that array, getting a distance from point A to latlong[0], A to latlong[1], etc...
//when that distance is >= user supplied distance, STOP, and draw a polyline from point A through the latlong points in the array latlong[78]
function computeTotalDistance(result) {
var total = 0;
var myroute = result.routes[0];
if(myroute)
{
//create a LatLon from the Starting point
var objGeo = new LatLon(Geo.parseDMS(myroute.overview_path[0].Qa), Geo.parseDMS(myroute.overview_path[0].Ra));
//call get distance from the starting point to each other point in the array
//each subsequent point should be a longer distance
var arrPointsToDraw =[];
for(var i = 1; i<=myroute.overview_path.length-1;i++)
{
try
{
var objGeo2 = new LatLon(Geo.parseDMS(myroute.overview_path[i].Qa), Geo.parseDMS(myroute.overview_path[i].Ra));
}
catch(err)
{
alert(err.description);
}
//here, total = kilometers
total = objGeo.distanceTo(objGeo2,3);
//add coordinates to our array of points that we are going to draw
arrPointsToDraw.push(new google.maps.LatLng(objGeo2._lat, objGeo2._lon));
if(parseInt(total) > parseInt(distance.value))
{
arrPointsToDraw.pop();//remove the last element of the array
break;
}
}
//at this point, arrPointsToDraw[] contains the lat/long points that are closest to our distance
//without going over
lines[lines.length] = new google.maps.Polyline({
path: arrPointsToDraw,
strokeColor: '#1589FF',
strokeOpacity: 1.0,
strokeWeight: 3
});
lines[lines.length-1].setMap(map);
}//end if(myRoute)
}
This code makes use of two fantastic collections of functions found here