With the caveats that this is just my very subjective experience, I'm not sure what you mean by "moderately active" or "an athlete", and I'm probably taking your 80/20 more literally than you intended:
I agree there's a lot of improvement from that first 20% of effort (or change in habits or time or whatever), but I think it's much less than than 80% of the value. Like, say 0% effort is the 1-2 hours/week of walking I need do to get to work and buy groceries and stuff, 20% is 2-3 hours of walking + 1-2 hours at the gym or riding a bike, and 100% is 12 hours/week of structured training on a bicycle. I think 20% gets me maybe 40-50% of the benefit for doing stuff that requires thinking clearly that 100% gets me. Where the diminishing returns really kick in is around 6-8 hours/week of structured training (so 60%?), which seems to get me about 80-90% of the benefit.
That said: Anecdotally, I seem to need more intense exercise than a lot of people. Low-to-moderate intensity exercise, even in significant quantity, has a weirdly small effect on my mood and my (subjectively judged by me) cognitive ability.
You are both more correct than me, my wording was quite strong because I just felt like a discussion of the importance of a non-sendentary lifestyle for improvements in thinking was completely unadressed on the site. Hopefully someone else can read more carefully and help us to optimize based on our many varied characteristics. Thanks for the emotional dampening!
You learn something about yourself when you push hard that you cannot learn anywhere else, and you learn even more from doing it repeatedly and consistently. Moderately active in relative terms is sedentary in absolute terms anyways.
Who is this a defense from?
Do ‘non-athletic thinkers’ regularly criticize ‘athletic thinkers’ in your experience?
Non-athletic thinkers are myopic. I see at least three very important reasons any rationalist must value exercise.
As someone who has trained extremely hard in distance running, sports in general and athletic conditioning in particular create an intuitive understanding of the fallaciousness of Cartesian Dualism and the accuracy of materialism in the sense that the mind is merely a part of the body.
Physical challenges also force one to understand the limitations of Kahneman's "system two." For example, one may know what it means to not start a race too quickly and then "die" (running jargon for running out of steam prematurely), but repeated failures in actual races teach one to realize the limitations of one's rationality, especially under stress (even if distance running is classified as eustress rather than distress; remember, cortisol levels & arousal are the same in either case).
Conditioning has been shown to increase cognitive performance. Results of a quick google finds: This study notes better reaction time in fit adolescents (no causal link, n = 30). This review shows decreased "system 2" aging in physically fit elderly individuals (causality likely, preponderance of evidence). Acute exercise (<1 hour) has also been shown to cause immediate improvements in decision making. I recommend a deep dive into the mechanistic research into the causes of this for structuralists. But any Bayesian looking to be more rational ought to make an update and start working out.
(Edit: Grammar)