Most of what I've read about fasting says that the metabolism will return to your normal rate once eating begins again. For me, exercise has been the key for years now. Working out REGULARLY is the sure-fire way to increase one's metabolism. Also, once you stop fasting, don't forget to keep up the water intake!
Whatever happens to the metabolism, remember that you just did a great thing for your body! Just the colon cleaning alone is well worth it.