As the others have pointed out, the 24 ohms you calculate from 120/5 is the total loop impedance of the circuit, including copper conductors, xfmr winding, and the resistance of the two ground connections.
Assuming a relatively short copper path, the bulk of the loop impedance will consist of the resistance areas or voltage-gradient areas of the two ground rods. So long as the two rods are far enough apart that their gradients do not overlap, increasing the distance between them makes very little difference.
