Clients often give me that WTF look when I make the following suggestion:
“Breathe only through your nose.”
Like, ok, what the heck are you talking about? And what does that have to do with the half marathon I want to run or simply beginning a running program?
“Well, just... everything.”
Faster Recovery and Less Injury
Nose breathing forces you to slow down naturally, turning running into a classic form of “self-limiting exercise”. If you allow your breathing to guide your pace, you will find that you can't run very fast because your nostrils simply can't move the same volume of air as your mouth.
The average person tends to do the easy runs too fast – nose breathing slows 'em down. It also improves oxygen concentration in the blood after exercise.
If you want to learn more about why slower running is critical for marathoners and beginners alike, click here. The bottom line is that running slower reduces injury risk BIG TIME and will allow for faster recovery between sessions.
Better Oxygen Quality
The natural filtration provided by the nostrils means cleaner, more humid air reaches your lungs. Think that might be a good thing?
Still sucking air in through your mouth? Look what else mouth breathers tend to enjoy:
Better Core Function
Nose breathing tends to facilitate proper diaphragm function. In other words, you're able to belly breathe more easily than with your mouth open.
Your diaphragm is not just a respirator muscle, but also a powerful stabilizer. As part of your “inner core” your diaphragm plays a critical role in spinal stabilization and as a runner, this is actually a huge deal. See, when your diaphragm isn't functioning as well as it could be your body will compensate by using all sorts of other muscles to play the role of spinal stabilizer.
Let's consider some common running injuries that can many times, on some level, be traced back to poor core functioning... That's right, nearly all of them!
Trust me. It's a big deal and I spend much of my initial time with new clients re-establishing proper breathing patterns and diaphragm function. (Keep reading to learn how)
Relaxation (and look, a lower number on your heart rate monitor!!!!!)
Lots of people I know attempt to use running as stress relief. Yet, if you're gasping for air with every stride you're missing out on feeling even better. Proper breathing is linked to your nervous system which regulates how amped up or calm you feel. Inhale through your nose, notice your belly expand outward 360 degrees, feel more relaxed immediately.
Oh hey – what's a major key to running faster? Relaxing. If you can't relax at a slow pace, what do you think will happen when you speed up?
It's kinda like lifting weights. If your form sucks when you deadlift 45lbs, adding weight will not make it look any better!
Putting It Into Practice
Hopefully that all sounds great. But where do you start? Do you just try to run with your mouth closed and hope for the best?
Well, sure, please do try that. Just expect it to take several tries before it feels “natural”. It IS a natural thing, but many of us have been 'chest' breathing since we were teenagers. This is something you'll have to commit to work on consciously. Here's an interesting workout idea that is based around focusing on your breath.
For some, switching to nose breathing and using a nice belly breathing action will take some thought, but they can do it. Others, however, will feel so uncoordinated that they'll get frustrated and give up on this whole breathing thing.
If breathing properly is just not clicking, start with crocodile breathing. Lie face down on the floor, with your forehead resting on your hands. Breathe through your nose and feel the expansion happening in your abdomen. You should NOT feel your shoulders rising (towards your ears) and falling.
The idea is to start in a very stable position (lying on the floor), breathe well there, then progress to a slightly less stable position (up on all fours). Soon you'll be able to breathe well while standing ...and eventually running.
Let Progress Come To You; Don't Force It
There is a deeply ingrained mindset of “no pain, no gain” in our culture. I encourage you to let go of that mindset. 
When it comes to training, it is wise to coax the body to fitness, not bludgeon it.
In my experience, clients who adopt a good nose breathing habit see consistent, long term improvement.
Those that try to rush, that lack patience, that jump from 6 week program promoted by a magazine to the next, well, they're the one's flaming out, feeling banged up, and looking for other forms of exercise because “running isn't for them”.
So I wanna know: have you tried running while breathing sole through your nose? What do you notice (give me “the good and the bad”)?