6'4" and 165lbs (193cm/74.8kg) seems quite thin though - I don't think you are necessarily providing the counterexample you think you are here? Maybe I'm misunderstanding your point. Internet BMI calculator says that would make you "underweight" and put you in the 7th percentile for your height and weight (as in, 93% of people as tall as you weigh more than you).
I'm not saying I wasn't thin. I definitely was. However, you wouldn't have been able to pick me out from a crowd as a 'cross country runner'. And to be competitive at that level, you can't be carrying around any extra weight. While BMI can be a helpful tool, it's pretty flawed. It doesn't take into account muscle/fat ratio. I really don't have much body fat, but am now only 5 pounds away from being 'overweight' accordingly to BMI.
Really, my point was that running 15-20 miles a week won't give you the body of a Kenyan Olympian. Running can be incredibly rewarding, I was simply trying to state that if I had to run 90 miles a week to be barely underweight, 15 miles a week shouldn't scare anyone off if they're just trying to be healthier.
