I have been DM'ing a six player group playing Pathfinder for the past fifteen months. The amount of prep and conversion time is almost enough to make me want to quit and start over. I run the game every other Sunday and the group just hit level 6. I figure I have another two years to go before the campaign ends.
I wish I had based the campaign around an Adventure Path; I would love to not have to spend ten hours between each session converting monsters, developing encounters, plotting the storylines, and brainstorming. The best part of the AP is that it has already done most of that; the DM doesn't need to spend quite as much time prepping between game session.
The issue isn't with adventures... its with a single adventure that covers 20 levels. To solve your situation, you could just pick up 2-5 smaller adventures and tie them together as you wished. My campaigns would regularly use commercial 'modules' that I would then embellish to make them a consistent storyline.