Creating a Campaign for your players is a daunting task. The Dungeon Master Guide's from every edition can help you get it started.
I have found with making different campaigns that it isn't about where you start, or where it is suppose to end, but rather what you want your players to gather along the way. Sometimes knowing the many great things you hope them to achieve is more important that knowing the exact geography of a low part of some trench. Whether or not you actually have all the information to support a given region that you players hope to explore is not important. All you should have about every region (to start that is) is what lies their that makes it different from other regions. Give it a story that needs explaining, solving, or deciphering. Your players will appreciate that more than the fact that the mountain is 11, 321 feet tall or that the forest covers over 300 acres or that the average tree height is 153.35 feet tall.
I guess the main point to gain from this is that you need to give the players something to think about while they travel. Give them something that won't go in one ear and out the other. Give them something that they won't need to roll a history check to remember .
Timothy Robert Swain