I'd like to decrease the number of game hours it takes for a Fast Travel journey to be completed. Is there a setting that controls this?

I don't know of a way to control it directly, but in case it's helpful: the global setting Timescale affects it, so in theory you could temporarily change the Timescale to a smaller number right before fast-travel, and then back to what it was when you're done.

That would be a good workaround if I can't find a direct way to change it.

Is there a way I can tell when the player has started or finished Fast Travel?

You might want to have a look at http://www.tesnexus.com/downloads/file.php?id=27352. It might not do exactly what you're looking for, but it might give you some ideas... I notice on Nexus there's another one called dynamic timescale too.

I think this is exactly what you are looking for: http://www.tesnexus.com/downloads/file.php?id=10525

Thanks guys, these will give me something to look at.