Looking good!!!!....Nothing beats the real thing... you could try taking a walk down a dusty forest track or farm road and scoop up a couple of bags of that stuff, (it"s that sandy brown/greyish stuff you need) Then spread white pva glue quite thickly on the road area and drop the stuff you collected on top (thickly) give it around 10/15 minutes to sink in to the glue, then tilt the board to let the surplus slide of the road.
Then if you"ve timed it right you cans tamp it down with the end of an old paint brush or make wheel or tank tracks with an old model, if the glue oozes through no panic just sprinkle a little more dirt with an old spoon and make the tracks again till you are happy..Let it dry 24 hours and your tracks will have hardened like concrete, then if you wish you can drybrush with enamels or oils to highlight for effect.. ..have fun !!!.
Steve