Exercise helps your brain think better due to physiological reasons, bit it gives you nothing different than every moment of contemplation gives anyway.
Every moment is a time to contemplate where one is and what one is doing.
There is always a challenge or a contrast tied to what you are doing. If you are lying on the beach relaxed, you are probably doing so because it is in contrast to sitting in an office and thinking hard.
However if someone forced you to lay on the beach all day, every day (even with bathroom breaks, food etc), you might consider it uncomfortable before long or even tortuous.
Excessive excercise provides nothing different from this. Anything in excess is a challenge. Every concept has an opposite concept.
Budda went through that idea, and found that balance was the true enlightenment. Of course, what that balance is, and how to approach it, is up to you.
I don’t believe in “no pain, no gain,” because it has no reference point.
For example, if you are training to be a pro-athlete, you are training for a sport that likely involves a lot of pain, because of the endurance factor. Therefore it is a necessity to experience this same pain in your training.
It is a necessity in the sense that you must feel pain if you CHOOSE to endure pain.
However, the real question is, “Why choose to endure pain?” It is nonsensical.
People do these things in order to compete against others, but this is also nonsensical.
You are where you are right now, because you believe that this is the way it should be. If you want pain, you can have it, but unless it is fun, it seems fairly pointless.