One of the things most enjoyable about writing, to me, is creating. Making a world. Or at least a sliver of one. A whole world would be almost inconcievable, of course. Even the most well realized alternate worlds are pastiches of our world, liberally spiced with innovation but still closely hewing to what we know. It can't be much different, or else the reader wouldn't have anything to latch on to, the very words being used wouldn't have much meaning. But still, if one creates a world, it's a good thing.
Then there's using our own world, and merely (I say merely, as if it were easy) depicting some portion of it well and deeply. Still, even in dealing with real people and real places, one is making them over, recreating them. It's different, but an act of creation all the same.
The amount of work and effort is much the same either way, to be honest. There's more structure and less freedom in setting a story in a known place; you have to obey rules that are already determined for you before you begin, and you have to be true, or truthy at least, to known constants. But you can just look up those constants, and you have things to build on reasonably easily. Making something from whole cloth really lets your creativity run wild, and there are no rules but what you set up for yourself, which is nice. Only you have to make it all up; there's no further authority, nothing you can consult but your own muse. You end up, either way, with problems, and with delights.
I'm in the way of creating anew right now. Doing some serious making up, while, as noted, dragging bits of our world, our history, this or that culture, along with me and trying to put them in where they might be of use. Sometimes the bits fit well, and sometimes a little less well, and you try to make it all work. I've done the other way too, and I like it, but it's not as liberating. You get the sense when making it all up that really anything could happen. (That's not true, as I noted. Not anything at least. Many strange things, and wonderful, but not any and all.)
I guess I'm about 20% to the length I'd like to get to. Longer is fine, but shorter wouldn't really thrill me. And at a fifth of the work, I'd think it would be good to have some idea what I was aiming for, where the end might eventually be. As a writer I don't like going too far down that road, knowing secrets and endings. I like to be surprised as much as the next guy. But I usually have a general idea of what I want to get to, and I don't know even that much yet. There are so many options.
I'm concerned it will get away from me. That I'll end up someplace too terribly weird, or too terribly banal. That I'll build up too much junk that neither I nor any reader could care about. That I'll grow bored with the lack of action, or too much action, and just stop.
Memory suggests that I often have these fears, and I've often gotten by them without much trouble. So here's hoping.