Writing a story a day is going to be a huge challenge. Inventing characters and settings and inhabiting them for just one day? Huge.
Don’t try to do too much.
We don’t have the time or space to tell wandering epics.
We have time for one incident or one central character or theme [1. by the way all of this is also not true. In writing rules are made to be broken. Except that one about the apostrophe. I will hunt you down and smack your palm with a ruler if you put an apostrophe before the “s” in a plural!]
If your story starts to wander towards an interesting side character, slap that character’s hand and promise him he can be the hero of tomorrow’s story. If you find yourself backtracking to show too much of what happened before the ‘now’ of your story, file the idea and write a prequel tomorrow.
The beauty of writing aevery day is that you don’t have to do it all today. You can write tomorrow. In fact, you have to!
Finish Today, Plan For Tomorrow
So finish the story you started (even if you’ve fallen out of love with it) and make note of all the other ideas that were so good they butted in today.
Good writers are those who keep the language efficient. That is to say, keep it accurate, keep it clear.