I have a friend who switched from a couch potato lifetsyle about 2 yrs ago, and has been cycling 10 miles/day and more at weekends. She can do 70 miles/day without any preparation , and is fairly fit. She started off at 300lbs, but has stabilised at about 200.
I dont think its very wise to try and force your body into a shape or size its not happy at. Getting fit and healthy is very different from getting into a size 10.