Treadmills are self-defeating because if you just stand beside the person running on the treadmill, you can keep up a conversation without moving your feet at all. The scenery doesn't change. It also often seems that treadmills are hosued in closed spaces too, and the air-quality can get a little muggy in comparison to the outdoors, although indoors might be preferable in Bangkok training times.
To get a start at the front row for the fun runs and marathons, and be classed as A grade, you need to put in sub-40 minute 10 km times consistently. That means you're running 250m every minute, one kilometre in four minutes, and fifteen kilometres in an hour. Personally, 20km was about the longest I could keep up that pace, but bear in mind that involves hillclimbs and traffic crossings that you don't get on treadmills. If you're on a treadmill, you should be able to go all day. I mean you can keep a glass of water right there for if you get thirsty.
Now, with this damned illness, I don't need no treadmill at all to feel like dying. It's an everyday natural gift, but I'd rather run in the street when I get healthy again.