The Long Game
When you sit down to play Fate, you might just play a single session. That’s a viable way to play the game, but let’s assume that you want it to go a bit longer. What you need, then, is an arc.
An arc is a complete storyline with its own themes, situations, antagonists, innocent bystanders, and endgame, told in the span of a few scenarios (somewhere between two and five, usually). You don’t need to have everything planned out (in fact, you probably shouldn’t, given that no meticulously planned story ever survives contact with the players), but you need to have an idea of where things begin and end, and what might happen in the middle.
To make a fictional analogy, an arc is a lot like a single book. It tells its own story and ends when it’s done; you provide some form of closure and move on. Sometimes you move on to another story, and sometimes your book is just the first in a series of books. That’s when you have a campaign.
When you have multiple arcs that are connected and told in a sequence, and that have an overarching story or theme that runs through all of them, you have a campaign. Campaigns are long, taking months or even years to complete (if you ever do).
Of course, that doesn’t need to be as scary as it might sound. Yes, a campaign is long and large and complex. You don’t, however, need to come up with the whole thing at once. As with an arc, you may have an idea where it begins and ends (and that can be helpful), but you really only need to plan an arc at a time.
See, the players are so prone to shaking things up and changing things on you that planning more than one arc at a time is often frustrating and futile. Planning the second arc of a campaign based on the events of the first arc, how it turned out, and what your players did, though...well, that can make for very satisfying play.