I'm a little worried because I got it in GTS, and I've hatched like 5 other, newer eggs and this one still isn't even twitching.
The egg hatching speed depends on the Pokemon that is in the egg. Some Pokemon take longer to hatch than others. You just need to walk a certain number of steps to hatch the egg.
A list of all the different Egg Cycles for each Pokemon can be found here.
You can also have a Pokemon with the Magma Armor or Flame Body abilities in your party to speed up the hatching process. Check your Egg's Summary screen regularly to see how close it is to hatching.
Hope that has helped you out!