Make continent-scale heightmap based on existing coastline

[Naima]

I'm trying to make a realistic height map based on a fantasy sketch map. I know the shape of the coastline, the size, and details at the level of "there are mountains here" and "there are plains here". I know the locations of some rivers, and some other features, but there's plenty of scope to add additional stuff.

What software can I use that can:

1. Work at that scale (I need a map of size about 1500x1500 miles)
2. Let me start with my known coastline
3. Will help me build mountains in the places I want them, etc, adding realistic erosion.

I don't need 3D rendering. (Well, I want to do some, but I'm happy to do that in a different program if necessary -- I definitely want access to the 2D height map either way!)

I make my maps maximum size 24kx10k more or less and If i go above I can't handle in PS due to my PC specs I guess, I use Photoshop , worldmachine, wilbur, fractal terrains too.