Depending on how far in the distance you're going all you might need is a terrain with some noise. That's how I made all the 'forrest' in this image beyond the 1st visible ridge: 
http://www.renderosity.com/mod/gallery/index.php?image_id=266155 

It's literally a terrain with some noise. Probably I added noise, blurred it, added a little more noise, etc. a couple times. The texture was quite noisy also. Due to the haze there wasn't much need for detail so the main thing to make things look believable was picking the same colors as the trees closer to the camera.

This particular trick definitely doesn't work for every case but it does render very quickly. 

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