I've lost 20 pounds in 6 weeks. I think my metabolism right now is about 800 calories per day. Yesterday for example, I only ate 700 calories and I ran the fastest 2 mile I've run in 20 years and yet I still gained .5 pounds. My strategy is to slowly eat more and more calories per day until I am able to eat 2000 calories per day. My question is: does anyone know how long it takes to rebuild your metabolism.