mental effects of exercise

It's commonly known that exercise helps with many mental disorders and improves one's state of mind. I'm posting to address how it specifically affects me. 

Mild exercise: Anything that does not make me sweat or breathe heavily, anything that takes less than 15 minutes, anything that does not take significant mental or physical effort:

Effects:  Mild exercise barely changes my state of mind at all. Once I have done mild exercise, I feel roughly the same as I did before. 

Moderate exercise: This is something that makes me sweat, breathe heavily, and feel moderate pain. It requires mental and physical effort. 

Effect: After moderate exercise, I feel better, sometimes only slightly, and sometimes nearly euphoric. I also experience a pleasing sensation of soreness in my legs. I don't understand why this is pleasant. 

Intense exercise: Something like a competitive cross country race or hard cardio workout. Requires a large amount of mental effort.  

Effects: After initially feeling sick, I experience strong euphoria, anxiety is gone, and I can't think very clearly or focus. I am indifferent to many things that would normally bother me. 

The euphoria is known to result from the endorphins released in exercise.

I am not sure why mild exercise doesn't make me feel better at all while moderate exercise often makes me feel much better. One possibility is that part of the feeling of well-being I experience comes from toxins being released from my body, and so sweating (not present in mild exercise) is required.

I'm also not sure why the feeling of euphoria is accompanied by a lack of mental competence (lack of focus) after intense exercise. One possible explanation is that since I usually don't eat for a while before intense exercise, my body is low on actual nourishment and is at the same time being flooded by endorphins, so my brain can't function well but still feels good. 

I'm interested in other people's experiences about the mental effects of exercise. Feel free to comment with links to your own posts on this or other informative sites.

Possibly useful resources:

DLPA for Chronic Pain & Depression - Health Educator Report #5

Pleasure Connection: How Endorphins Affect Our Health and Happiness

The Opiate Receptors