Hi again.
I think those little bits you're describing about zoning out in school, or while doing homework, or while being forced to sit until you eat your veggies (for me it was fish, but yeah) is like daydreaming. Particularly when you're a child this is very common.
You probably don't even remember where your mind went at these times, and that is also common, as you are not really consciously thinking at these times. It is like your brain is so severely underwhelmed that it just doesn't process what is going through it.
With the running. I think you've answered your own question.
Dissociation is when your body, and mind, becomes stressed to the point that it cannot handle what is happening, and you end up going away somewhere in your mind so as not to deal with it. This is something that people learn to do when they're experiencing repeated, or extremely severe, abuse. I think you have to go into a shocky state for it to happen...
It is like your mind just snaps right out of it, and you're no longer present for whatever is hurting you. When this happens you will have no recollection of what was happening to you at the time, and it will just seem like you lost a chunk of time.
Those examples you gave could be considered dissociation to a point I guess, but I don't believe that it is actually dissociation.
My last episode of dissociation happened when I was thrown into a stressful situation that I wasn't sure how to manage. The incident is explained in my diary titled "Anniversary, Alone, and Flashing Back" if you'd like to go read it,the title is "Rat loses tail, I lose my mind" it may help to put real dissociation into context for you.
Dissociation definitely does stop a person from going insane, it is like the ulimate defense mechanism. However, as far as I know it requires quite a bit of stress before you do it, and then, it just happens, you have no control over it. However, I believe that once you do it, and do it several times, it may be easier to go into dissociation. It will require less and less stress to get you to that point. Still, it requires more than just the average daily stress.
If you do not dissociate, it is a good thing. It is proof that you're able to handle the stressors in your life in other, more productive, ways. There are many other helpful defense mechanisms that people have, and go through, before they have to dissociate to save their sanity (or even life).
I am sure you experience some or all of these others, and as I mentioned before, the behaviour where you take on your abusers personality, and push people away, is probably one of these.
You don't have to worry that you'll go insane if you don't dissociate. I think if you're ever up against something severe enough to cause your mind to break (go insane right there in that moment) that you will dissociate...I wouldn't worry on it overmuch.
Good luck finding the answers you seek.