Congratulations... I am running Second Darkness and it is the first AP i have run for my group... It is turning into a fun one. Just a couple of things that have helped me a lot. Many of these reflect the fact that i'm not really quick at making stuff up on the fly
1) Get a list of names - i use baby name lists off the internet. For this setting eastern european names seem to fit nicely. Have 20 male and female names at your fingertips, so when you need to make any situation real, you don't have to invent a name.
2) I keep a binder section for locations with bare bones info (name of business, proprietor). You'll probably want to have some bars, inns and shops prenamed and scattered throughout town. This makes it much less obvious which direction a party should go and lets them spend more time exploring the city.
3) Someone posted a timeline. It is awesome. I adapted it, moved a few events around and added some "non events" so it wasn't always a meta game clue when the PC's were asked to run an errand, or when someone came into the Gold Goblin (it is also a great way to foreshadow minor characters that will appear later or in the AP). The fact is that weeks go by as the story unfolds, so it really helps to know when it is and when payday is so you can keep the flow and trickle out clues and details appropriately. This has been the hardest part for me/my group we are all used to the sort of "time is of the essence, hurry up" type of campaign where there is no room to stray from the tracks. In this it is all about straying from the tracks and figuring out bit by bit how things fit.
4) Riddleport IS random encounters. Many of them (con men, drug dealers, fences, prostitutes) are the flavor of the town. Be ready for them so they feel genuine to the players. This is a fun city to GM, lots of great potential for characters with interesting motives. My players are writing an entirely different story even as the adventure is unfolding, which is great fun.
Most importantly... ENJOY IT! The really nice thing about the beginning of this adventure is things evolve slowly. You have all the time in the world to set the scene and make it believable. The burden of pacing, at least at the start, is on the players. The trick is to keep the days moving at a steady pace. We have gone whole sessions without combat encounters, and yet there are enough odd and interesting things happening that my players are enjoying it.
Have fun and let us know how it goes!
Arnold