Because the eye is used to seeing complexity in landscape, it does take awhile to create something believable. I usually create the base map at around 2048 square within Carrara, apply the filters and rough painting I want and then convert it to a pixel image and export it. I can then refine in Photoshop and reimport to Carrara and apply additional filters like erosion. 

Be aware that you may need/want a 4096 map for up close landscapes but a 1024 map for items in the distance. Landscape scenes are usually made up of several terrain maps hidden by camera angles. ;-)