So far, every single (legitimate) response has addressed training issues. That's fine, but it leaves much undiscussed.
I'm prepared to be dissed, mocked, laughed out of town with a probe up my rectum for this, but I've got a question: Does running on a treadmill even begin to feed the soul in the way that runnng outdoors does?
I've used a treadmill maybe a dozen times. It's good for a lot of things--including making adjustments to my stride, avoiding treacherous icy roads, etc.--but it always leaves me feeling like a lab rat. And that's not why I run. I'm not cultivating that feeling.
I run to get OFF the treadmill for at least a part of my day. Running is a primal thing. I don't need to tell anybody on this board about that. Humans spend 10,000 years as endurance predators, chasing down our prey. Something deep in us likes to head out the door and make loops--big loops, smaller loops--in the world. Circle the lake. Circle the park. Circumnavigate the five boroughs. Or go out and back.
I like to get off the grid. I like to take my chances out there. It keeps my soul alive.
Go ahead and sneer. Tell me running isn't primal. Laugh off my claims and get back on the treadmill and do the lab-rat boogie. Next time I see you in a race, I'll chase you down and eat you. Then you'll know primal.
I'm exaggerating to make a point, but only slightly. I've got absolutely no problem with runners who make occasional strategic retreats to the treadmill. But I feel sorry, frankly, for any runner who doesn't feel, somewhere deep down, that treadmill running lacks something on the soul-level--something that heading out the door and actually TAKING A RUN supplies.
Read Bruce Wilshire's WILD HUNGER: THE PRIMAL ROOTS OF MODERN ADDICTION. It's about running through the woods, tracking with your dog, being startled by a deer, and many other things.
So what does treadmill running lack? Here are a couple of things:
1) It disarticulates running from the fact of forward motion--from the fact of actually propelling yourself through space and imaginitively recreating yourself in three dimensions as you do so. It's a little like disarticulating the idea of eating from the idea of nourishing yourself. In soul-terms--if not in training terms--treadmill running is junk food. Please read that sentence again. I'm not talking about training effects. I'm talking about spiritual profit and loss.
2) If you want, it's a playground for narcissism. If you do your TM running in front of a mirror, you get to stare into the pool, Narcissus, and lose yourself in your own glorious body--rather than, for example, engaging the World Out There, with all of its pleasures and challenges. Hmm. Welcome to the modern "health club."
3) It deprives you of the possibility of truly engaging with another runner--pursuing, catching, passing, accidentally jostling, tracking, etc. These are primal things. They're part of why we race. The thrill of racing. And they're a subconscious part of why many of us head out the door. (Note to TeenBoy: No, I am NOT talking about stalking. Please don't start with me.)
Track running, although it may seem to diminish running--in my terms--just as much as treadmill running, is in fact quite differerent. Track running allows for all three things I've just listed. In spiritual terms, it's real running. Treadmill running, in spiritual terms, is junk food. I like junk food from time to time. Cheez Doodles. But I don't for a minute fool myself into thinking that I'm nourishing myself.