From a blog I read:
Every runner I've encountered who averages more than 20 miles per day shuffles. That kind of energy expenditure requires the most efficient running style possible. The top ultrarunners are almost always shufflers. Though some runners can shuffle very fast (Alberto Salazar could shuffle sub-5 minute miles seemingly forever), it is not the best running style for short distances. I learned how to shuffle, only to discover that it's not good for running through tree roots - more on that next post.
Efficiency is important at every distance, but means different things for different races. Most top runners in any given race have remarkably similar running styles. There are exceptions and they are almost all due to some biomechanical fault being compensated for. Bill Rodgers ran with his right arm making much larger looping motions than his left; he had one leg slightly shorter than the other and the arm motion helped with balance. Jim Ryun, the great miler, swiveled his head like a bobblehead doll; he didn't need to, it just happened when he ran relaxed and it didn't effect his speed.
I have an unusual arm swing - I can be picked out of a crowd of runners from a long way away. That awkwardness decreases the faster I run; when sprinting, my form is nearly flawless. For me, the odd arm swing is comfortable, but it leads to other form problems over long distances, making me very inefficient after several hours.
The bottom line: the more your style looks like a sprinter's, the better you'll be at sprinting. The more it looks like an ultramarathoner, the better you'll be at ultras. If your running style gets better the faster you go, the shorter the race you should run; the better it gets the slower you go, the longer the race you should run. It sounds obvious, but few ever pay attention to it.