If you are not yet using this, you should. It is called gmap pedometer.
This is a web-based application where you first enter a starting point on a map that you drill down to by zip code or place name. Then you enter points along your route. All the while you see the cumultative total of miles; push pins markers appear on the map at each way point and each mile.
It lends itself best to road running, but it has a handly little button where you can select the option they call "manually (straight lines)" where you can plan an off road (i.e., trail) route.
This alternative causes you to enter a bunch more points to create any curves or turns--you are actually entering a lot of small straight line segments--but that's only a matter of some extra clicks. It works rather well.
One tip: you start your route by clicking "Start Recording" and each intermediate point you must double click.
Give it a try: you will be very pleased with this tool!