So is it something like Runners High?
After long running or training, you come in a special mood where you get aware of a power that your mind isn't working anymore. You keep on running on the power without thinking.
I will, respectfully, say "No" to this analogy.
If "Ki" were merely an endorphin rush, it could not serve much of a role in martial power or effectiveness.
I don't claim to be able to hurl balls of psychic force through my fingertips or lift a car to save a trapped baby or even do the kinds of internal skill manipulations of my body discussed at length elsewhere on this form beyond some basic exercises. (I recommend you check out the archives about "ki" and "kokyu," BTW).
But I have done endurance sports for thirty years, and Aikido for a dozen and counting. The endorphin effect is pleasant, and may help someone find a "second wind" by reducing the experience of suffering. Not much else really.
To me, the question illustrates Mary's point.