Katabatic and ewolin are so right about proper technique--one should hold the paddle with only enough grip to keep it from actually falling out of your hands. And often open the high, pushing hand, splaying the fingers so that the paddle loom rests lightly on the base of the thumb where it meets the forefinger. This, plus proper torso rotation, and varying your tempo and your paddle angle, will keep you paddling comfortably all day.