Running a modern game takes 1-3 hours of prep for every 1 hour of play. It's a lot shorter if you are experienced. But that's the point.
If you want to play a modern D&D that covers all 3 pillars, hits the aspects of the PC's race/class/background, and offers an exciting setting or narrative, it takes a long time to prep for a newish DM wh isn't trained set by step.
If you want to play an old school style game, a game that is PC agnostic, or uses a simple setting or narrative, it only takes a swift training to prep for a newish DM who isn't trained step by step..