Most people misunderstand physical activity.
They see a superb kayaker and think "they must be really strong"
or if they try it themselves and struggle and get tired they think "I'm too weak".
Muscles have almost nothing to do with it.
The difference between a pro and an amateur is almost entirely
the neural pathways that subconsciously control muscle firing.
A beginner athlete is using way more force than necessary to do movements,
and is firing lots of muscles that don't actually help the movement at all,
and is even firing lots of muscles that *resist* the movement.
(one of the more extreme cases of this I've experienced is punching;
almost everyone very strongly resists their own punch; in a
straight punch like a jab the bicep should be almost completely relaxed).
A pro athlete is firing exactly the right muscles at exactly the right time,
and no other muscles. This allows them to achieve the same motion with far
less effort. In a sport that also has technique (like kayaking), it is
almost entirely the technique and the neural pathways that make the difference;
in fact you can see people who are superb at some physical task
(kayaking, tennis, whatever) and seem to be able to do it for hours
but are obviously in quite bad shape.
In fact as your technique gets better, if you want to keep getting a good work out,
you have to keep pushing yourself harder, which can be difficult mentally.