I finished C25K back in August, and I agree with Nancy that your next steps (so to speak) depend on your goals and what you're hoping to accomplish.
By the end of the program, I also wasn't running 5k in the half-hour, so I added a couple of minutes each week (about a 10% increase) until I got to 5k. Once I got to that distance, I started mixing up my runs a little with some speed work (fartleks) and new routes. I also bought the app "Zombies, Run!," which I highly recommend--it is super-fun!
As you keep up your running, you will definitely start to notice improvements, particularly if you incorporate speed work into your runs--as they say, if you want to run faster, you have to run faster. :)
|218 Days until: Chicago Marathon