I find 40 hours is much more productive if spread over several weeks. Maybe one's unconscious works on it the rest of the time.
If he could permit himself to start over several times, I can too.
Is it possible to have a "clear but poorly-defined goal", or are you having a joke? "Fun" is an emergent property - recognizable, measurable but the creation isn't formalizable (though BTW your link mentioned something about game grammars...)
Poorly-defined, meaning: one man's fun is another man's boredom, it's hard to pin down what exactly "fun" is. Thankfully, though, there are many good educated guesses, so we can't act like "fun" is impossible to define.