Congrats on the successful first run! Like you have mentioned, I will concur with the assessment that if they want to get together earlier, then its a sign that you gave them a good session.
As a GM in other systems, I can certainly understand your fear of the Matrix rules and such, but I've usually found that when in doubt, winging it tends to work rather well. If you don't know a rule off the top of your head, make something up and look it up later. Usually, your players won't be sure either, and if they are, they can probably turn to it in the books pretty quickly.
That being said, I've never tried GM'ing Shadowrun yet, mainly because I have a great GM already, but also because I have to admit that I would likely struggle coming up with opponents, etc. I like the idea of monster levels in other games -- but then Shadowrun doesn't use levels even for the PCs so that wouldn't help much here.
Anyhoo, congrats again and best of luck in the future. Oh, and if you are concerned with time (particularly prep time since I know that can be a huge hang up for GMs) you may want to take a look at the Missions adventures. I've played through one now (and started a second) as a player and they aren't too bad. It also seems as though they would be fairly easy to adjust (pretty sure my GM was adjusting things on the fly as we started to take everything that wasn't nailed down).
They do give you relatively short adventures (which is good for Shadowrun) and you don't HAVE to report them if you don't want. The only thing you might need to be careful with is that they are generally going to be geared toward convention play, which means certain types of characters might not do real well in some of the Missions. As a Rigger, I was a bit disappointed during the first run when I discovered that I wasn't really going to be able to use most of my drones (driving an SUV and bringing armed drones through a department store doesn't exactly work)
, but it was made better in the end with a nice loot haul (which I am assuming was adjudicated on the fly).