Things that helped me when I started GMing (and still see constant use):
-Pre-staged mooks, conflicts, and layouts. Sometimes the PCs just aren't buying what you're selling, or are going after it in a way you never anticipated. That's not a problem, if you have some general opposition already prepped in your back pocket. A page of potential random conflicts (not all violent, just things the Runners can Run/run into), some general NPC mooks that can be whipped out with a name change, and a short list of ways they can run into the group on another paper. This is, essentially, the more group tailored "random run generator". But when you have to pull this out, you really have to consider...
-Breaks. Never be afraid to take a break. If the players just went to left field instead of the hockey rink, and you resolved the immediate consequences, feel free to take a break to plan your next actions. 5 minutes planning something out (even using the random run generator) and you can have your storytelling feet under you again and ready to push the story forward in a new direction. But sometimes...
-If you run out of material, ideas, or steam you should be ready to stop the session. If it's been fun, and everyone has enjoyed themselves, but it's 3 hours into what would normally be 6 hours and you have nothing left to give then you should feel fine calling a stop. Enjoy talking to your friends about the weather, sports, your lives. Myself, I prefer getting more ideas for what to run next time, which means...
-Feel free to talk to your players about the story, their characters, your NPCs and the game. You don't have to give away anything you're planning, but it's amazing how much information you can get from players about what would make it fun for them. I've had my whole group tell me that while they're having a blast, they really didn't think one of the characters had really been in the spotlight yet, and they couldn't wait to see what that character could really do. I've also had a really tactical, combat focused group tell me that they didn't feel like they're characters were really engaged with the community. These were things I hadn't noticed from behind the scenes, where I saw the big picture.
So, in short:
-Have something prepped for those random times. Nothing fancy, just a go-to grab bag of fun or tension
-Take a break when you need to (but preferably after resolving the actions you're in the middle of at the moment)
-Don't feel obligated to go beyond your limits. If that's all you could give, then it's already 100% effort on your part
-Talk to your players. They are the greatest resource any GM could have, beyond any books, forums, movies, or stats.
Of course, you should always remember why this game is called a game: It's ALL for FUN!