The reason you have trouble breathing when your run is because you use larger muscle groups more vigorously than when you walk.  You have to ease your lungs into running.  Try this little trumpet player trick: before your activity, take as big a breath in as you can. Fill up from the deepest part of your abdomen all the way to the top of your lungs.  Hold that breath in, and while you do, do stretches; arm stretches, torso stretches, leg stretches. Keep that breath in, and take periodic 'sips' of air.  When you feel like you can't hold the breath any longer, slowly breathe out the entirety of the air.  Do this exercise twice, then begin your warmup walk.  During your warmup walk, make sure your breath is rhythmic, and you are breathing as deep as you can.

Another trick you could try anytime is timed breathing: give yourself a moderate beat or click in your head and take a deep breath in in 8 beats and out for 8 beats while raising and lowering your arms with the breath(this makes sure you are expanding your lungs and diaphragm properly). Then do the same thing for 4 counts, 16 counts, 2 counts, longer and shorter, switching it up as you see fit.  Maker sure when you do this, you are filling your lungs completely not just taking little breaths then make sure you are emptying your lungs completely as well.

These are a couple of exercises that I have used in my years as a trumpet player to expand my lung capacity.  Don't forget though that you have to ease yourself into running.  Only run as far as you can, or use a training program.  Only increase your milage by 10% a week or you risk injury.  Hope these exercises help.  They mould make more sense if I could demonstrate them and the proper way to breathe (yes there is a proper way to breathe), but that just isn't possible in a message board!

I struggle with feeling out of breath also.  I've been running for about a year but lost condition over the winter.

Some things that have helped me are:

1.  Stay relaxed through your upper body as you run.  Don't get tense and stiffen up through your shoulders, upper back, or neck.  If your rib cage can expand as you breathe, you will take in more air.

2.  Every now and then, deliberately take a complete breath that really fills your whole core down to the waist.

3.  Do a deliberately strong exhale, followed by a deliberately deep inhale.  Many of us never completely exhale, strangely enough!  Shallow breathing can lead you to that hyperventilating feeling, light-headed and almost faint.  Deeper breathing can help prevent that.

4.  Finally, you probably know this already, but wind sprints are great for increasing your aerobic fitness.  Go to a local football field (when no one is around!) and sprint as fast as you can from the goalpost to the 50-yeard line.  Walk back.  Repeat.  Work up to 10 sprint/return sets.  You'll see a big benefit very quickly.

5.  When running, take a little walk break when you need to.   Keep moving briskly but just let your lungs get caught up and heart rate down a bit.  As time goes on you will need these less and less.

Keep on running!!

SO glad to see this post.  I too am very new to running and this is my biggest struggle, and very discouraging.  Matter of fact I joke that I am going "running" and always put it in quotes, as I stop so frequently due to breathing that it is not really running.  Really interested in if the "sprints" exercise helps. Please keep us posted!