The Life of the World [TK] I write in my notebooks all the time--pages of dense scrawl, barely legible even to me. As you can tell from how infrequently I post, I judge very little of it good enough for publication, but I keep at it regardless.
My notebooks are not journals--frequently I will be the only person to read them, but they are all addressed to some theoretically "you." The most heartbreaking thing I come across when paging through my old notebooks are orphaned introductions--half a notebook page of set-up and preamble, cut short by the train arriving at my destination, or the end of my lunch break, or something shiny flitting around the periphery of my vision. Trying to catch "you" up, I lose track of what I was trying to say in the first place.
In college, I had to write all papers in one sitting; if I left I'd lose the thread and would have to retype everything I'd previously written to get it back. This may be why I've never written a novel beyond the first chapter.
Except for the fact that I can't handwrite as fast as I can type, I like this method. My notebook comes everywhere with me. Having a writing method that can't multitask also cuts down on distractions. The best writing process advice I can remember receiving came from Cory Doctorow, adapting a journalists' trick: Rather than stopping what you're doing to research a fact, write [tk] in the place. "tk" is a letter combination almost unheard of in the English language ("Atkins" is about the only remotely common word to have it), so you can do a Search through your text once finished with writing to find it and research it at the end. This is of course for simple facts like distances, heights, and the like, not for fundamental bits that you wouldn't be able to proceed with if you don't know how they work intimately. But this little trick will keep you from researching inessential facts that might send you tumbling down the Wiki hole.
As fun as it is to get lost in research, and as fruitful as it can be to your imagination, sometimes you eventually realize that you've been stuck researching troop movements through Scotland in the mid 1740s for the last two hours.