Snapping polyline to roads - google-maps-api-3

Question

So I am trying to snap my polyline to roads using WALKING travel mode, currently I have a WORKING polyline that updates when the geo position of the user moves, then then removes the old poly line from the old location and plots a new one from new origin to destination, however this is a straight, "as the crow flys" line.
function polyLine(myPos){
var start;
if (!start){
start = geoMarker.position;
}else{
start = myPos;
}
var end = marker.position;
var pathCoords = [start, end];
var pathGo = new google.maps.Polyline({
path: pathCoords,
map: googleMap,
strokeColor: "#FF0000",
strokeOpacity: 0.7,
strokeWeight: 2
});
polyLineArray.push(pathGo);
}
I also tried experimenting with various code snippets I found via other threads here but couldn't get any to work. This is what I'm currently playing with.
function calcDist(myPos) {
var start;
if (!start){
start = geoMarker.position;
}else{
start = myPos;
}
var end = marker.position;
var request = {
origin: start,
destination: end,
travelMode: google.maps.TravelMode.WALKING
};
directionsService.route(request, function(response, status) {
if (status == google.maps.DirectionsStatus.OK)
{
var steps = response.routes[0].legs[0].steps;
for(var step = 0; step < steps.length; step++)
{
polylineOptions = {
map: googleMap,
strokeColor: "#FF0000",
strokeOpacity: 0.7,
strokeWeight: 2,
path: steps[step].path
}
drawPoly = new google.maps.Polyline(polylineOptions);
}
drawPoly.getPath().push(ble);
}
});
}
So I am wondering whether my syntax is correct for the walking directions, the function is pulling across all the coords successfully as it uses same variables as polyLine(), yet no line is drawn at all on calcDist
Thanks.
EDIT
Doing something similar to the Google Maps documentation, still no luck
var directionsDisplay;
var directionsService = new google.maps.DirectionsService();
function calcDist(myPos) {
var start;
if (!start){
start = geoMarker.position;
}else{
start = myPos;
}
var end = marker.position;
var request = {
origin:start,
destination:end,
travelMode: google.maps.DirectionsTravelMode.WALKING
};
directionsService.route(request, function(response, status) {
if (status == google.maps.DirectionsStatus.OK) {
directionsDisplay.setDirections(response);
}
});
}