1. My training is pretty basic. When on the couch, I try to relax, concentrate on the breath, play with awareness, investigate sensations, thoughts, etc.
When I feel I'm out of "focus", again I concentrate on the breath, and focus on jhana factors. Mostly sustained effort,
one-pointedness, joy.
Sometimes I narrow the focus to the tip of the nose. Sometime I focus on the raising and falling of the abdomen.
Once I achieved some concentration, I look for the 3 characteristics. I really try to stay in the present, to try to "catch" things as they are happening. To break the "continuous" sometimes I do a repetitive mental note like "present", "present", "present".
Once the mind "speeds up", I focus on the observer, or the suffering.
My advice, just go and try to do things, put effort and play.
Don't compare so much, only Arnold is Arnold and your experience with meditation will be different from mine and everybody else.
2. It's not at the rational level. It's the way what you see, feel, think, your sense of space are processed.
My guess is that all the sensations are "post-processed" on two directives (I have control of them or not), the famous fight or flight, control or surrender.
Don't think too much about who you are. Think about what you control and what you don't control. About causes and consequenses. On who does the things you do, and why do you do them. Try to catch the sensations in the body that trigger actions.
When in doubt, remember MCTB: "there are no special sensations that are in unique control of other sensations", "sensations are not fundamentally split off from other sensations ocurring at the moment".
For this I recommend noting, fast noting.
3. Meditation brings relaxation. Over time, that relaxation has a positive impact on the body.
Other things that have impacted the body have been more indirect.
For example, meditation brings more awareness of the body, so you tend to talke care of it better.
Also, with meditation you are better able to cope with pain and doubts, very useful things when training the body.
4. Categorization is done in order to know if fight or flight.
The fight or flight (control or surrender) is rooted in a person (the one who fights of flights, or the one that controls or surrenders). That is my guess.
Things do look different, very different after path.
The best I can say is that if you stare at something after a path, you know that something that was keeping you attached to it is gone, some mental filter to your senses that required effort is gone. And it's not subtle, it's really very noticeable.
I don't know if "worth it" is enough, "life changing worth it" is more accurate. Eventually words have limits, for the real thing, you'll have to find for yourself

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