Breathing is something that is done naturally until we pay attention to it, what's amazing is the number of paddlers that hold their breath.
Unfortunately this does not help your paddling or your body as you deprive your cells of oxygen causing you to fatigue more quickly. You tell yourself I need to get fitter, but you could try breathing more effectively, give these two exercises a go
1. BLOWING UP A BALLOON
Blowing up a balloon can teach you what it feels like to have a complete and full exhalation using the abdominal muscles. Take a deep inhale and exhale with a long steady forceful breath and contract the abdominal muscles to force the last of the air out of your lungs
2. WALL SIT WITH DEEP BREATHING
In the wall sit position, reach your arms forward and practice long slow deep breathing. Take a deep breath in and then exhale completely using a 5 count to expel as much air as possible. As your spine is against the wall and in a neutral position the breathing exercise will force you to use your diaphragm and absorb to perform the inhalation/exhalation and not rely on the spine neck or shoulders to expand the rib cage.
Bonus - you will get a great quad burn too