I don't know about what's standard or best for a beginner, but when I started it never crossed my mind to have a weekly mileage goal. There were too many other things to work on and improve, and regarding distance it was mainly going a little further, or a little faster. I'd pick something to work on and concentrate mainly on that for a few days or a week, then pick something else. Bike handling, holding a straight line. Smooth and fluid pedal stroke, never coasting. Breathing. Consciously higher cadence for a while, or sometimes lower in a harder gear. When the rides got to be longer, I'd focus sometimes on position and posture, and keeping relaxed, things that show up when you spend more time in the saddle.