When I DM my only major planning happens at the start of the campaign, I dream up some cool NPC's of varying level and roles (with lots of random rolls to give me some playing in-character challenge), I draw up my original game path idea, they will be hired by this guy to go do this. Then the random improve begins, the first time I ever DM'd the group took a total alternate root to what I was expecting and I had to flush like 5 hours of prep down the drain. I also try to set up a unique homebrew world for them to inhabit, my current project is Ravenborne. After this initial set up prep, I run them through my only 100% designed dungeon, the level 1 icebreaker, from then on, I let them encounter NPC's and brainstorm quick ideas for what they want to pursue, my icebreaker always has loads of loose ends for them to attempt to tie up! I can quickly get them on path, and the things they do, the way their characters come to life, their passions and goals are enough fuel for my brain to come up with interesting story arcs that can keep the whole party entertained, I include lots of brain teasers and logic puzzles and award them XP based on their performance dealing with them, as constantly just hacking up monsters gets to be mundane.