I agree with Eden that a wheel upgrade is a good place to start. You might look into getting a set of handbuilt wheels, which depending on the build can be cheaper than factory wheelsets but still provide a marked improvement in ride quality, weight, etc. An upgraded rear derailleur will also likely improve your shifting.
That said, I wouldn't necessarily encourage you to upgrade any of your other components just to upgrade. Instead, I would suggest either saving your pennies for a new bike down the road or waiting until something actually wears out--which it will with enough riding. This is coming from someone who upgraded things on her own bike. I don't regret doing it, but over time, I've gotten a better sense of just how expensive it is to maintain my bike. It adds up.
Good luck with your century!
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher