Fun read! I always felt that the lack of a heart beat was the hardest part of a zombie to explain. The best I could think of was that the infectious agent somehow supplied energy anaerobically. It would break down host cells at an accelerated rate with a unique metabolic pathway not reliant on oxygen. This still would be less efficient than aerobic respiration, but that might explain why classic zombies are so slow and why their bodies can look so decayed. Heck, it could also explain the hunger for human flesh as that could provide more energy and stave off decay.