A 28-year-old woman from Delhi presents with a 2-year history of recurrent episodes of severe muscle pain and weakness triggered by sustained exercise or fasting. During an acute episode, her serum creatine kinase is 8,500 U/L (normal <200), myoglobin is elevated, and urine is dark brown. A muscle biopsy shows subsarcolemmal glycogen accumulation. Genetic testing identifies a homozygous mutation in the PFKM gene (phosphofructokinase-muscle isoform). Which of the following best explains why her symptoms are exacerbated during sustained exercise?