Not sure about all that, but meditating a full half hour at first can be a problem for some people. The biggest barrier is that you often fall asleep, or you spend the whole time wondering if you can stop meditating, or wondering how long it has been since you started.
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Posture is another problem if such a thing concerns you, especially if you sit on the floor. Beginners often cannot get into the full Lotus which helps with posture and comfort. Keeping a straight back is nearly impossible for most without being in agony. If you start at 5 minutes day and use a wall to lean on when you get sore, your back will get stronger gradually. If you go too far you will just injure yourself.
Everything depends on the type of meditation as well; for insight or concentration meditation, the longer you do it, the faster you will get better at it, no matter the level of experience. Still, 5 minutes is a good place to start if you are worried about your own willpower or abilities. If you jump into it with too much force for your will, you may lose interest entirely.
Here is a good place to learn about meditation from the monks; more than you ever wanted to know:
Scroll down to the meditation section