I've worked on the terrain a bit more and made the mountain ranges thinner like suggested.
Here's an image of the altitudes:

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And here's one with more natural colours:

Click image for larger version. 

Name:	Terrain 1a.jpg 
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Next I want to keep working on the terrain. I realise that I need smoother plains, more coastal lowlands etc. How would I best create these? Would this kind of process work, using lots of incise flow and fill basins?

If anyone has any tips or criticisms then they'd be much appreciated. I want to get the terrain sorted before I move onto working out the climate.