I've posted a few times on this. I read an article a year or two ago that said women don't burn fat the same way men do b/c of the child bearing thing. The basic thesis was that women's bodies have evolved to store enough fat to support a fetus for 9 months due to natural disasters like famines & such.
The article concluded that, b/c of the above, after approximately 60 minutes of cardiovascular exercise it becomes anaerobic & you stop burning fat.
I don't know if this is true or not, I'm just passing along the info. It seems to make sense but our bodies are strange machines so who knows. Also the article only mentioned time & not frequency. I don't think there's anything wrong w/doing cardio as much as you want, other than the fact that sometimes your body just needs a break. I do know I've found 45 minutes of cardio 4 times a week to be plenty for me! ;-)
Also a lot of runners are training for something specific, so maybe they run every day to increase endurance or to shave a few seconds of their mile? Just a guess--I run sometimes but I've never trained for a 5K or a marathon or anything.