Rollers actually require even pedal strokes, in addition to a stable upper body. But, above all, I would say concentrate on pedaling smoothly, and on pedaling in circles. Once you work on getting an even pedal stroke, the rest falls into place. Pretty soon you'll be riding no hands.

It's difficult to explain, so perhaps someone else will chime in. Rollers are really a great way to get rid of pedal mashing. Also, once you get back out on the road after training on rollers for a while, you'll be able to hold your line better and navigate a pack better.

I don't know about miles. I can really only go about an hour to an hour and a half before parts start going numb. Plus the concentration required to stay on rollers a long time is pretty exhausting. I did make it two hours once, watching a movie.