I used to fast travel all the time, and I often still do fast travel if I see tons of things I don't want to pass through(like three feral trolls close enough together so that they will know if I attack one, but far enough apart that my fireballs don't hit all of them), and can't spend all day going cross country.
Point in case: Mouth Of The Panther to Kvatch.
I used to fast travel this path, and I knew that it took me a few hours more then I'd want to spend, so I once decided that I needed to rush one day, since one of my quests said I had three days to do some stuff before it was too late to do it. I knew that the fastest way to get somewhere is it just go straight, and I looked at my map, and sure enough, the fastest route was right over the water. I cast a few spells, and booked it across the lake on my horse and just kept running, not even stopping to fight the big ass spiders attacking me(stupid paralyze spell they have, being ranged and potent). It took me maybe three hours to get there, and that's with time scale 30. I even had time to chat with the NPCs there, stock up on repair hammers, and dive through the gate there before the day was over. I didn't take the same route back, since I fast traveled, and even though it was just a bit after midnight, when I got back to where I started, it was mid day(I think. Didn't check the clocks well enough, really).