I know you have already accepted Mark's answer, but here is an idea, from a writer that starts with rather vague story plans:
While you are in love with your idea, write down what you love about it. Why is this such a great idea? What scenes are you imagining? What funny or astounding or terrifying moments will this idea deliver? What kinds of problems does this make the characters face? Will people die? Will love be lost or gained? What is going to excite characters inside this idea, to make them move and take action? Is it money, or power, or terror, or a weeping-at-night desire to save the poor shovel-head turtle from extinction?
Before you start writing an actual scene, or thinking about a hero, write yourself a touchstone: As many pages as you can muster on why this idea grabbed your imagination. Write this to be the cheerleader for your future self.
If in some weeks or months you grow to doubt yourself, read it again. See where you went wrong. Maybe the cheerleader can convince you to back up a bit and fix the story.
If you get bored with the writing, it may be because your characters are making obvious, predictable and safe choices. They are boring to read about!
The story is not advancing, or advancing too slowly. You need to find where that started and find some dynamite to throw. Something that forces them off that boring path.
Stephen King had this problem with The Stand, he was stuck for weeks and thought he'd lose the novel 3/4 of the way through. To fix it he literally threw dynamite in there, he wove in a back-story of sabotage that blew up and killed half his characters and sent the rest reeling on a completely different path. Reading about that sabotage (without knowing it was an expertly done add-in) created a sense of dread leading up to the explosion, it kept seeming the saboteurs would get caught, but they kept (quite logically) escaping and it looked like they were going to get away with it. Which they did.
There is a thin path to walk that keeps your characters in character while sustaining the suspense of what is coming, and their journey to it compelling. It is easy to take a step off that path, and wander for a few days or weeks. If you get bored, that is what happened: go find the last bit you enjoyed writing, and try to change the game so you don't wind up in the same place.