Not trying to discourage you but tracks are a lot of work, especially if you don't have the right tools/equipment. Do you have access to a dozer? How big of a tractor and (chisel) plow do you have? And the biggest question, where are you getting all your dirt from? Like Mike said, it takes many many truck loads. Did you watch the SX race at Dodger Stadium, that took ~500 truck loads.
A mixture of clay for the faces will keep them from becoming rutted out as fast.
Also isn't that track a little "aggressive", especially the middle step-on-off section. That'll take a lot of maintenance to keep nice, same with the whoops. I'd suggest keeping it simple and starting out with just some doubles (tables would be better but they take to much dirt) and 180's.