A 28-year-old male athlete presents to the sports medicine clinic after completing a high-intensity interval training session. He reports muscle fatigue and soreness. Blood work shows: lactate 8.2 mmol/L (normal <2), pyruvate 0.15 mmol/L, glucose 92 mg/dL, and normal liver function tests. After 2 hours of rest, lactate normalizes. Which of the following best explains the metabolic fate of the elevated lactate during recovery?