Why can hypoglycemia cause visual problems and aching muscles?  

Both need sugar. Both muscles and the brain need sugar to work well.
Glucose and VIsion. Glucose is the energy currency for all tissues. It provides energy for all cells. If the sugar levels are low, the vision may be blurry or focusing may be difficult. Lack of glucose results in rapid depletion of atp in the muscles, thus causing Lactic Acid buildup and soreness.