This is just my workflow, so might not be the best but it has worked for me. (Not free, but there are demos of all the programs so you can test it out, I’ll put links below)
1:
First thing you do is obviously get your 2d map setup the way you like it. You already have this step but here is an example
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2:
What I do then is take it into photoshop and create a stamp that can be used in mudbox.
3:
Then take it into mudbox, make a plane, and apply the stamp. Here you do your editing and any major features you want in the landscape.
Then you export a height-map from there.
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4:
I then take the height-map export into World Machine for erosion and splat mapping export. (Although I would not recommend it, for testing you can get a plugin for world machine called “UDK layers”. Eventually you will want to create your own extractor macro to get the maps you need 
but it’s good for a quick mock up.
5:
After you have everything looking the way you want it, you export out your splat maps. [Link to a tutorial that will help your export]](A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums) That tutorial will walk you through importing and getting started on your in game tweaking!
When you do it this way, you can edit the landscape in engine with the built in tools while you fill out the landscape :)
Links to the software, there should be demos of everything:
http://www.adobe.com/products/photoshop.html
Hope it helps a little bit it’s a general overview, but if you have any questions feel free to PM me or add me on skype