While I agree with pretty much everything jsimms435 wrote, I have to say this; for every rule there's an exception.
Personally I avoid dreadmills like the plague, however, I know two folks who trained exclusively on treadmills over this last winter (it was pretty rough up here in New England); both individuals logged between 70 and 90 miles per week on the mill, and both competed in the Boston Marathon and logged times in the low 3-hour range in spite of the fact they are both within a few years of being 60.