The first time I realized I could just give players a level whenever I felt it was appropriate for the story, I almost fell over.
One of the biggest pitfalls for novice game masters of any tabletop RPG is knowing how and when to say no to players.
I haven't been running as many TTRPG games lately as I'm used to, so when I got to run our latest D&D Spectacular, I very deliberately went into it throwing everything I possibly could at the players.
Here's a little secret -- I don't do a lot of formal planning for running my D&D games.
I remember when I decided to start my first Dungeons and Dragons game as a DM, and found about a month into it that I'd accidentally created my own campaign world.
If there's one thing that every Dungeons and Dragons game needs, it's a Dungeon Master.
I've been playing and Game Mastering various tabletop RPGs for a long time -- from old school Dungeons and Dragons and Pathfinder, to Wraith the Oblivion, Exalted, and Talislanta. I've run GURPS, Champions, Silver Age Sentinels, Unknown Armies, Conspiracy X, Call of Cthulhu, Earthdawn, Heavy Gear... it's a lot of games.
You've been DMing for a while now, let's suppose.
Let's set the stage.
There are a lot of excellent Dungeons & Dragons setting books out there that will give you all the details you need to run a game in their setting, and those settings (also known as campaign settings, world settings, campaign worlds and/or simply campaigns) are often excellent.