Since I'm a fan of immersive trailing/exploration based maps, I design the landscape first (sometimes I also use 3rd-party heightmaps for a more natural base terrain) and then load it raw into the game (no distributions, plants, etc.) so that I can get an idea for scale, then drive around it a bit. This gives me an idea of where trails/paths would go if it was a real-world location (instead of trying to go OVER that huge cliff, a real trail would go AROUND it).
Once I figure out where the trails would fit best, then the imagination comes out as far as visual design, obstacle/mud/water placement, etc. Try to keep the trails interesting, give the player some neat scenery to enjoy while they follow the trail, throw some rocks or mud or something that makes sense in there so they're not just drooling along on autopilot to get to that next watchpoint or garage.
Then I'll load up the map again with trails (still no trees and stuff yet) to look for weird terrain issues, and to get a better visual idea of how the trails "interact" with the terrain. Sharp turn in the trail? Why? Avoiding a huge boulder/tree/ditch? Any spots where a trail doesn't quite follow a valley like you hoped it did? Broad-strokes style troubleshooting.
Once the trails have been inspected and fine tuned, that's when I start on the real details like trees, grass, etc. More from a visual/aesthetic standpoint here vs functionality as addressed previously.
This is where a lot of people can get bogged down and lose sight of "progression vs perfection" and have a hard time saying "it's finished enough for release."
Hope this helps, but by no means am I an authority on the subject! lol