Insert clever title here.
Many people I know cannot get into running because they spend the entire time thinking consciously about how much it hurts, like not being able to sleep because you can't stop concentrating on your own breathing. My wife, for example has to make little deals with herself the entire time. If she runs three miles, she does it by telling herself "ok when I get to the next tree I can rest", and then, when she gets to the tree, instead of resting, she makes another little bargain "ok, just to the telephone pole, then I can rest" (FOR THREE MILES). A friend of mine can play basketball for hours, sprinting up and down the court. But he can't run even a mile without stoping, he's just too focused on the act of running. How tired it's making him each second, how much further he thinks he can go.
This would drive anybody nuts. And it's the main thing I think that makes people just starting out decide that they hate to run, without really giving it a chance. I'm pretty heavily INTP, so for me, the act of 'vegging" comes naturally. If I have any sort of down-time at all, traffic, waiting in line, sitting in useless meetings, etc.. I pretty quickly turn inward and start mentally working on problems, or inventing new ones, going through checklists of stuff I wanted to get done, daydreaming, mental meandering, whatever. I think for people who aren't good at vegging, they think they need something else to take their minds off running, while they're running. Music helps, so many, get an mp3 player, others like treadmills for just this reason, they can watch tv.
I think however that anyone can learn to vege if they want to, and I submit that it's a mentally healthy thing to do. In The Muse In The Machine (one of my very favorite pop psychology/AI books), David Gelernter describes conscious thought in terms of mental focus. Highly focused thoughts are the conscious ones. They're the ones most of us have when we start running. "Man this hurts... Man this still hurts... Yep still hurting". Low Focus thoughts are the ones that just sort of pop up, seemingly from nowhere as a result of the fact that you're not really thinking /about/ anything at all. You're just ... vegging.
Gelernter thinks that low focus thinking is something we need to do a lot more of. He says that lowering our mental focus allows us to pull in seemingly unrelated thoughts, memories and emotions and put them together to form breakthrough ideas in a very "stream of consciousness" manner. You smell something that leads to a memory, which leads to another, which is the perfect solution to the problem you're having at work. I'm probably murdering his idea, but to the extent that the above is an accurate description, I agree with him completely because it happens to me all the time.
This low focus mental state, where I can just stream thoughts however I need to, hits me just about after the 1 mile marker. Right when I'm getting warm and settling in. When I'm running daily it's absolutely the best part of my day, and it's therapeutic no matter when it happens. If I run in the morning, I get ideas, and epiphanys, and solutions to problems I didn't think were solvable, I can focus better at work, and pound out code. If I run in the evening, I get a mental toilet flush (for lack of a better metaphor), a cleaning out of all the distracting sand, within which I finally spot the flecks of gold, things to try tomorrow, and obvious dead ends to stop pursuing.
If you give long distance running a shot, I think you'll find that this mental state comes quite naturally after a few weeks or so. I don't think it's learned behavior, it's something that we can all do, and contrary to what you might think, the act of running facilitates it. You just need to get over the proverbial hump, and throw out the headphones.
Which brings me to the subject of treadmills. I've owned a treadmill for quite sometime now, and used them for a good while before that. Since at least 1993. Never once in my running career have I ever been able to vege on a treadmill. Maybe it's just me, but there's something about the lack of scenery, or maybe the static digital displays in front of me, but I can't get into a low focus state of mind. If you can manage to run outside (like you don't live in new jersey or whatever), and you're just starting out, I HIGHLY recommend pounding pavement over treadmills at the gym. That low focus mental state is probably the single biggest reason I run. In fact when I say that I "love to run", the mental enema is like 9/10's of what I'm trying to express. It's not something you want to miss out on.