Initially it will zoom to your location. Once this is complete, scroll around the map, and press once, or long press on a location to add a marker. Then add the markers to follow your proposed route (usually looping back to the start marker somehow). The total distance of the route will be plotted at the bottom of the screen.
For extra information, click any of the markers to view the distance up to that point.Additionally, all the gestures available in google maps are also available.
Note: elevation is not currently taken into account, this may be added in a future release.
Note: Please use responsibly, ensure the route you have chosen is suitable for your chosen activity (i.e. There are pavements/lights).