Deb, breathing through your nose tends to get people to breath more into the lower lobes of the lungs and to use their diaphragm more. That's a good thing, it improves oxygen exchange so your blood should be able pick up more oxygen per breath. But you can only do this up to a certain level. With practice, I can breath through my nose (both in and out) in my zone 1 and 2. By zone 3 I can breath in through my nose but I have to breath out through my mouth. Zone 4 is all through the mouth.

I like nose breathing, it helps me to monitor my effort level and forces me to be more efficient while I run. I don't think it's made me any faster though.