In Morrowind, I actually didn't always run, since running would decrease your fatigue, on the othe hand, since the walking speed was slower than Oblivion, walking everywhere simply took too long, so I'd usually alternate between running and walking. Run until my fatigue starts to get low, then toggle running off. In Oblivion, though, since running did not reduce fatigue, there was little reason to ever walk, unless you were decorating you're house since you'd need to control you're movements precisely to avoid messing up. I sometimes still walked in interiors in part for the sake of roleplaying and in part so that I wouldn't run into things too much but otherwise, it was typically run everywhere, except when sneaking, in which case I might walk rather than run when I'm close to enemies to ensure the least chance of being detected.
I suspect the way it will work in Skyrim though is that running and walking work more or less like in Oblivion, while sprinting decreases your stamina quickly, I could be mistaken, of course, but it seems to make sense, because sprinting is usually used for short bursts of speed, and in the past, many games that allowed it would have it drain stamina, so you could not sprint indefinitely. Though in some recent first person shooters, I've noticed no stamina at all, and instead the reason you don't sprint everywhere is that you can't attack while sprinting.
But really if you were a globe-trotting fantasy hero, you run everywhere....there is no time to waste.
I can't think of too many fantasy books or movies I'm familiar with where the character's are constantly running, and that makes sense, because the character's are traveling, not running a marathon. They don't want to waste their strength.
Of course, since this is a game, most players aren't going to want to walk constantly as it it would just take too long to get to the action.
That really doesn't render the issue irrelevant, as you won't fast travel all the time. Sure, you can fast travel to a town, but once you're in the town, you still need to go to the place in it you want to go to in real time.