Per novice:
I have good cardio from running/biking but I have never really been out
of breath while kayaking. I'm doing a bout 8.5 minute kilometers in 10
ft by 28 inch wide kayak.

Check into technique. With proper paddling technique, your arms are just used
to link the paddle to your torso - they don't do much work. The work gets done
by the muscles of the torso and legs.

Also, long term, think about looking into a surf ski. It's much more rewarding
in the sense that when you paddle harder you can feel it go faster - rather than
just chugging along at the limit of hull speed.
--
PeteCresswell