What value to you get in return for this significant investment of time? I've been curious about meditation for a while, but never really understood the benefits. Every time I've tried it, I've walked away cranky and uncomfortable.
No, it is hard for the mind to understand, since the mind is quiet at this point. The mind itself is the disturbance that prevents us from knowing what is there when it is no longer "veiling" what is.
This is also called being in the present moment, being in the Now, abiding as the I-am, abiding as awareness etc. My first recognition was of what was real and what was not. Only you can make that recognition, it cannot be told and proved by someone else.
The second recognition was of myself being the silent awareness inside which the mind and the "I" was playing. The whole play of the personality or "personal self" is seen through. The suffering of this mental being crumbles. Can you imagine the freedom ? No wealth or power can buy this.
Personally, I've always been drawn to practices that emphasize simplicity, ease, gentleness, and acceptance in the cultivation of awareness. If I need to pee, I get up and pee; if I need to adjust my position, I adjust my position. I don't force my mind, I do the opposite: I let go of my mind and instead watch it. Thoughts come and thoughts go, and a surprising number of them are pretty funny when I'm not caught up in them.
It's truly easy because you can't do it wrong: the goal isn't to have no thoughts, be perfectly present, or have super-concentration. You will have thoughts (lots of them at first), you will have moments when you're present and moments when you're not, you will have pretty crappy concentration at first. Over time, you develop a very easy, effortless form of concentration that doesn't involve forcing anything. Concentration isn't the goal, just a pleasant byproduct.
The benefits? I feel much more centered and present, more relaxed, my mind is sharper. My mood tends to be better. To be sure, there are times when I'm cranky when I start and cranky when I finish -- we don't get to use it to force our experience to change. But by practicing being gently present, practicing accepting "this is what my experience is right now," I seem to have more pleasant moods and fewer, briefer unpleasant moods.
As far as bladders, in the Zen tradition, half-day or full-day sits are short sessions of sitting (maybe 35 minutes) alternated with walking meditation (maybe 5 or 10 minutes). During the walking meditation, you have an opportunity to step out if you need the toilet. Also, there can be a meal and rest period thrown in so that it's not actually 4 or 8 hours of just sitting.