Dungeon World encourages GM improvisation, but does not discourage preparation
Dungeon World discourages an on-the-rails style of campaign where the players are simply there to work through the GM's plot.
In the GM section, the authors emphasize improvisation (to run Dungeon World you'll need to adapt to the decisions your players make as they move through the world) but Dungeon World is not purely about improvisation.
If you look in the Dungeon World materials you should see sections for Fronts and The World.
I feel like you either glossed over these sections or missed them entirely as they really serve as the foundation for creating adventures and plotlines in Dungeon World. Be sure to study them as they contain a lot of information and GM tools/mechanics, much too much for me to go over in a single answer.
Fronts are how you as a GM organize challenges, goals and risks to the players.
Fronts are secret tomes of GM knowledge. Each is a collection of linked dangers—threats to the characters specifically and to the people, places, and things the characters care about. It also includes one or more impending dooms, the horrible things that will happen without the characters’ intervention. “Fronts” comes, of course, from “fighting on two fronts” which is just where you want the characters to be—surrounded by threats, danger and adventure.
Fronts are built outside of active play. They’re the solo fun that you get to have between games—rubbing your hands and cackling evilly to yourself as you craft the foes with which to challenge your PCs. You may tweak or adjust your fronts during play (who knows when inspiration will strike?) but the meat of them comes from preparation between sessions - p. 185
There is even a helpful little checklist:
Creating Fronts
Here’s how a front comes together:
Choose campaign front or adventure front
Create 2-3 dangers
Choose an impending doom for each danger
Add grim portents (1-3 for an adventure front, 3-5 for the campaign front)
Write 1–3 stakes questions
List the general cast of the front
Page 187
Campaign Fronts represent overarching threats/plotlines between multiple sessions. Things cooking in the background while the adventures go on. "One Ring to Rule them All" serves as a good example of a campaign front from the Lord of the Rings.
Adventure Fronts occur in the here and now, they could be a place or a direct threat to the party. The Mines of Moria would make a great Adventure Front.
Dangers are the creatures, places, or things that constitute the threat of the Front and get to make moves just like monsters or players would (But only you the GM knows what moves they are making, the party only sees the effects).
Dungeon World's setting lends itself to all kinds of heroic stories
The generic fantasy setting of Dungeon World should be easy for your players to latch onto. It bears a lot of similarities to what they are used to with Pathfinder. Just because the world is similar to a Pathfinder/D&D style setting doesn't meant the adventures your players have are required to be just as similar in their approach. Exploring ruins can be less about killing monsters and avoiding deadly traps and more about puzzle solving and discovery. Saving a kingdom from military defeat doesn't mean the party single-handedly kills the opposing army, but maybe they manage to go boost morale, find new supplies, and help train new recruits.
The World section in the book goes heavily into how to create towns, cities, ruins, and wilderness for your players to experience, how the actions the players take and those they don't should affect the world, and how the world might be affected if various fronts resolve.
Anyone can attempt anything
The Dungeon World resolution mechanic of rolling 2d6 + mod with the static ranges of 6 or lower = Failure, 7-9 = Success with complications, and 10 or greater = Complete success means that characters can always partake in problem solving and trying new things. In fact Dungeon World rewards risk because the best way for characters to gain XP is by failing rolls.
