Your two questions clearly represent the very same anxiety that each of us has had before going and seeing it for ourselves. Each of the comments is valid, and there is nothing like the right kind of practice. You may be seated, probably standing, and certainly shooting through a small opening out into the light from an elevated or ground level blind. You will be inclined to overshoot, whether elevated, ground level or pit blind. Mock openings can be created with wood or paper or even cutting up an old sheet. The narrow opening does distort you depth of field. However practicing from a window, balcony, or down a steep stair case can be very helpful. When you get out into the blind you should not be bashful about taking a few shots at targets with field tips or judos in order to get the right feeling and to just get past the jitters. You won't need that every day but at the beginning it will help. There is a lot of pressure on not to miss because, among other things, there is a pretty hefty price tag around each animals neck. That is part of the experience whether you like it or not.
From the point of view of the shot distance, you can be pretty sure that a standard situation is going to be at or under 20 yards. Work to be comfortable in that zone and just don't take a shot that you are not comfortable with.