I find it easiest to plan a big campaign based on a main character. This gives you a beginning and an end and lets you map out where you want the PCs to go. From there you will run into questions about the usual;
What do they need?
Who will they meet?
Pre-roll some heavy encounters and fit them into the scheme of things. you know, make it all come together. By filing in some of the major and important pieces you can see how it all looks on paper.
When you see it like this you can see what details you need to work out. Common races and weather. Cities and towns, travelers and heroes. They will all come out with room to spare for in game atmosphere... which is what we all really need to be worrying about. Less details and more game atmosphere. Let it go where it goes