Built Environment

Anyone can explain, How to make environment at game like make land with same cave tree and any obstacles on it?I tried it with terrain and some object 3D on it. I imported tress.3ds,cow.ms3d and put it on terrain.But with this method i spent much resource to built only terrain,2 tree , and cow. So it seemed work slowly.whereas,  i'd like to make environment like RTS/RPG game. What the best way to make this environment?

Ok thx

Obviously doing it by code would be very slow. You need some sort of terrain/level editor to do that.