It's slow, but you can automatically teleport yourself across all the chunks in a specific area of the map to have them generated using Python and the pexpect module (which I've used to send the teleport commands).
First, make a copy of your game data for testing purposes, then open a command prompt at that directory and do the following:
$ virtualenv venv
$ source venv/bin/activate
$ pip install pexpect
Paste this code into teleport-expect.py (adjust playername and the xcoord and zcoord ranges):
# set this to your minecraft username
child = pexpect.spawn('java -Xms2048M -Xmx2048M -jar minecraft_server.jar nogui')
child.logfile = sys.stdout
child.expect('%s joined the game' % playername)
child.sendline('gamemode 1 %s' % playername)
for xcoord in range(1000000, 1005000, 16):
for zcoord in range(1000000, 1005000, 16):
child.sendline('tp %s %i 255 %i' % (playername, xcoord, zcoord))
child.expect('Teleported %s' % playername)
child.sendline('say all done!')
$ python teleport-expect.py
Once the server starts, login to the game. You should see your player automatically being teleported one chunk at a time across the area of interest. Visiting a 5000x5000 area will take multiple hours to run.
It's not a fast way to generate a map, but neat to see the scenery fly by. I mainly wanted to test running the Minecraft server inside a pexpect session. Lots of potential for other automation (say, watching for user-created commands on a vanilla server)!