Changing texture on prefab at runtime (c#)

I think I'm missing something fairly obvious here, which is entirely possible since I've only just begun with C# (having come from a fairly extensive Java background, I found the lack of strongly typed objects in javascript to be horribly limiting.)

I'm building an Sim style game that I want to randomly generate terrain using terrain cubes. Essentially, I want these cubes to inherit certain properties based on the game state - for example, if an actor walks over a certain piece of terrain multiple times, I want that terrain to develop into a "trail" and I want to change the look of that piece of terrain accordingly. As such, I have a prefab called TerrainCubePrefab with the properties, getters and setters that I need.

When I instantiate a TerrainCubePrefab, I can't seem to obtain a reference to it. Here's what I'm doing:

 terrainCube = Instantiate(Resources.Load("TerrainCubePrefab"), new Vector3(x,0,z), Quaternion.identity);

Hopefully that code is correct, as I don't have it right in front of me at the moment. "terrainCube" is declared at the top of the script, but does not give me any access to the terrainCubePrefab's methods in its scripts. I've attempted casting the call to a GameObject type, but that doesn't work

 terrainCube = (GameObject)Instantiate(Resources.Load("TerrainCubePrefab"), new Vector3(x,0,z), Quaternion.identity);

How do I go about obtaining a reference to change the texture that is applied to the prefab I've instantiated? In the prefab's start() method, I have it currently set to a default "Grass&Rock", so I've got that working properly.