Ditto from the creator of RTJP. Start small, maybe in a TC~Isle-sized level, and then work up from there.Also, a lesson from my massive JPDS project - don't make an area with such a large size that it will take forever to finish, if you are new to Tres modding.
Also, that heightmap is going to be extremely high-poly once converted to Tres, probably to the point that the engine will crash unless you do some serious optimization. You might want to turn the detail down a couple of notches before converting. Even my 4096x heightmap for RTJP which was heavily optimized several times has crashing problems, which forces you to spend time-consuming periods manually lowering the terrain polycount. If you only use a small amount of that heightmap's area, you can probably get around this by making distant areas low-detail as opposed to higher-detail ones for upclose terrain features - again, see Trespasser for this. Good examples are the streambed and rock steps in IJ, though the rock steps are extremely unoptimized.