One other thing I wanted to point out about nasal breathing is the number of breaths per minute. In the original article I found that turned me on to nasal breathing, the author recommends a rate of 15 breaths per minute. In Jack Daniels' "Daniels Running Formula", he estimates that top runners take 45 breaths per minute. This is quite a difference!
Many people have criticized running because it produces oxidative stress. However, this depends on the method a person breathes. I am not an expert on oxidative stress, but I would think that 45 breaths per minute over an extended period would create a lot more damage than 15 breaths per minute! This study(pdf) shows that males at rest breath at an average rate of 16 breaths per minute. Therefore, running with nasal breathing has roughly the same breathing rate as a person at rest (though I'm sure the running produces deeper breaths).
I have continued to stick with nasal breathing in my training. I can run 20 to 30 minutes on nasal breathing. I still slip up, but it's mostly due to losing focus rather than needing to breath through my mouth.
