there where things you could have doen pre hard mode to help your worlkd out, and also build some human made bioms because some of the run the risk of going away forever like jungle and desert bioms.
@blue, I didn't find a diffirnce in threats form the corruoption to the crimsion, if any thing crimsion at lest looked cooler XD ^_^
Actually two things i do is:
Go onto a non-hard mode world and get the stuff there.
and the other i mine a 5 block wide tunnel around a mushroom biome and the Hallow wont get to it! I just block the tunnels with dirt so enemy wont get through and the biome is safe from Hallow. woo!
Lot's of vile powder used to undo some damage to it and it is now a Hallow-proof zone.
(didn't mind me about the amount of vile 'shrooms I have
TAKE THAT HALLOW AND CORRUPTION!