After 40lbs bicep curls I notice some benefits, but its little more than trivial.

People knew that for ages !

I too neglected my health when in my early 30s, luckily at 35 I started exercising and running again. I got in the best shape of my life around 40 and it was the best decision I've ever made.

As for specific experiences, my creativity and productivity on the mornings when I go for a sunrise run, is off the charts. I also tend to ponder things when swimming on lunchtime break and get surprisingly (at least to myself) good ideas.

What I notice is that if I haven't been to the gym for a while, my brain becomes progressively resistant to effort. Since I have ADHD (diagnosed in childhood, not adult self-application of a wiki page on the subject), it is probably more pronounced than for a lot of others. But it is a very noticeable effect that provides incentive to not have prolonged pauses.