I want to experience what an all out 800 feels like

In my experience, the less prepared you are, the more it will hurt. All my best 800s, when I was in killer shape and had been knocking sessions out of the park, felt surprisingly easy. I crossed the finish line, saw a PR, and thought, "That's it? That's all it took?" Meanwhile, those first few races of the season might leave me incapacitated.

So if you wanna ride the lightning, go try it soon. Then do some specific work and try a few more times. Age and experience should have almost nothing to do with it.

A 6:00 flat 1500 guy as me is fast (""""fast""") enough to get to feel the sting the last 300-200 meters if the first lap is paced accordingly and really goes to the well from there?

I don't see why not. Effort is effort. The faster the first lap, the more the second will hurt. The only time it wouldn't send someone into the well is if they're totally untrained and unwilling to "push" through the pain.

Why can't I feel the "my legs don't work anymore" at the last 100 of a 400?

It's not because a lack of willingness to dig in, believe me.

Everybody hurts. Some people just run faster.

You'll feel pain if you go out @ equivalent 800m pace through 400. I feel like 800s feel ok enough through 500-600. The last 200 is brutal. The last 100 should feel like an all-out sprint but your legs feel like bricks & you feel like you're going backwards for the effort.