Terrains take awhile to get the hang of. The best trick with terrains is to not try and connect them together. This is true for Vue and other applications that have terrain functions. (Vue has method to create an infinite terrain procedurally but it hits render times pretty hard) Carrara has the option to create a tiling terrain which is good, but you will see the pattern if the camera gets above the scene much.

The most common tactic in terrain and environmental scenes (Vue, C4D, Carrara etc.) is to create multiple terrains and stagger and stretch them to disguise their edges - this requires attention to camera angles. 

Another trick is to create a couple of terrains and stack them so they intersect.

The surface replicator is your friend when populating terrains. Creating a custom distribution map is the way to create controlled chaos.

Carrara's terrain shader is extremely flexible and powerful. But all that power comes with the penalty of learning to control the complexity with the large array of options and controls. Tearing apart some of the terrain shaders that come with Carrara is a great way to do than. 

Just the crazed ravings of a lunatic madman....