Minecraft – How to load chunks without circling around the world

In this thread on Reddit, I found a command line script that will generate terrain by repeatedly restarting a server with varying spawn points until the area you specify is filled. This seems like just what you're looking for.

There's also a Bukkit plugin named WorldGenerationControl which can do it on a live server.

There are three straightforward ways that you can get the newly added worldgen from any mod to be present in your existing world.

• Go far enough away from your currently explored areas to cause the game to generate new chunks (this doesn't require using MCedit / WorldEdit)
• Use a version of MCedit that knows about (and won't replace with air) mod created blocks to delete already explored chunks which, when recreated, will contain the new worldgen. This has the downside of deleting anything you've build in the chunks, but if all the construction on the map is confined to a small area with the rest of the explored area effectively being 'wild' then it can effective.
• Use mystcraft to generate a new age. With the new age creation mechanics in the latest versions of mystcraft this is more complicated than in earlier versions but still fairly straightforward. The easiest way to create a new stable (vitally important) age for the purpose of getting access to new worldgen is to go into creative mode and get a 'creative spawned' mystcraft notebook that will automatically contain all symbols and then follow a guide such as this question to assemble the right symbols in the right order to get a stable age.