Good advice.
Sometimes the numbing can be caused from standing with too much stress in the legs. Relax your legs. Bend your knees a little. Change the position of your hips slightly.
One hour is always better than two half hours, but do what you can.
I always prefer hour sessions, because the energy becomes stronger with time. But, time is often the issue. Like this morning. 
Chunyi does say that some people will not feel the energy, but that is such a small percentage. Most people feel the energy between their hands during Breathing of the Universe.
