I'm not familiar with Campaign Cartographer, but I will give such advice/suggestions/questions as I can.
If the land area is twice the size of California, then the green region is a little bigger than California, which is 163,700 square miles. So the green is on the order of 180,000 to 200,000 square miles? Is there going to be anything of interest in the white area?
Generally, a rain shadow creates very dry and arid land on its side of the mountains. It doesn't really make sense to say that the area in the rain shadow is "still on the moist side". It would make more sense, I think, if the prevailing winds blew north/northeast/northwest over the land, dumping a lot of water near the coast, and then leaving a rain shadow on the north side of the mountains.
Here's a possible tectonics explanation. What are now the north and south halves used to be their own islands on opposite plates. The north one slammed into the south one, creating a big line of volcanoes. Now that the continental crust has joined, however, subduction is mostly finished and that explains why the volcanoes have not erupted for a long time. (Except I get the feeling that you're planning some sort of a volcanic cataclysm...so maybe there's enough magma left under the mountains for one final show.)
Rivers look mostly good, except for the two on either side of the gulf. Those two, and the right one in particular, come close to the gulf without emptying into it. This means that the land between it and the gulf is exceptionally high, which the map does not indicate. I would shift the rivers further away from the gulf. Also, are there any tributaries to be seen?
Is this going to be a Norse inspired map, at least with place names?