Sorry for the late response. I saw this when you posted, but I needed some time to answer.
As an MC, your goal is to convey a real world to your players, something they can feel part of and inhabit. In order to do this, you need to also make the npcs that inhabit your world real. In this way, the book offers you a variety of threat types that you can start to lean into, but these threat types are often generic, and following them literally will leave you with very standardized types of people in your world. Instead you can build your npcs much in the same way I suggested above for player characters. You first put them in the world, give them a name, ask questions to players or yourself to find out who they are and the thing it is they suffer from. That is their thing, their want, and you structure a couple rules of action around it. Then you network.
You take that NPC and you ask, who does this person know, care about, need, want, fear? Why? And you step into those people and you find out who they are by having them interact with the players. Or you tie two seperate npcs together with a third. This process takes place mostly on scene, and is informed on while you ponder in between sessions. I wonder why that npc reacted like that? I wonder what makes so-so a threat? Is their threat aimed at the players or someone else? It is probably driven by a need, a lack, a scarcity in their world.
Your next focus, or perhaps your first focus as an MC is to include your players. By involving them, you don't need to come up with everything on your own, you can form a collective story. In this way you can be surprised, and play right along side them. For this reason it is essential that you enjoy the characters your players are bringing to the table. You're suppose to be a fan of them, that's how you can tell the story in a way that helps them move forward, rather push them back. Some of the biggest struggles with groups are find those sweet spots where everyone is on board, so when you do come across them, remember them. Those are what you'll want to build from most.