Below only works if you work in real world scale:
If you press 'View' in the top menu and then click '3D Preview Location' then you can read the distance from camera in that window when pointing your mouse to the part of the terrain you want to have in focus.
In the blur tab of your camera you enter this distance from camera value.
From there it's a matter of knowing some photographical principles, like long focal lengths with large apertures have narrow depth of field the closer to the camera. That kind of stuff.
An intermediate focal length lens like 35mm does not offer narrow depth of field unless you are very close to the camera and have a large aperture, like f/1.4.
The sample Klaus supplied shows nicely how you can make it work when not using world scale values.
It also demonstrates that aperture values in the camera node come at a price.
I managed to shave off 25% render-time by setting aperture to 25mm (f/1.4 equivalent). This creates a smaller circle of confusion which takes less sampling.
However, since the scene was not real-world scale the depth of field effect disappeared.