"Heavy" Advice That's Tough To Take

It's not officially "official", but my doctor is ready to recommend/prescribe to me a complete change in lifestyle. In order to deal with my body weight being significantly too low, he is asking me to gain one pound a week for the next twenty weeks, and restrict my workout to only thirty minutes, three times a week, whether it's lifting weights or doing cardio. Start date: immediately.

1 pound of weight= an additional 3,500 calories to typical caloric intake

I guess it can be done, but I'm a cardio junkie who's got to have that daily endorphins rush. Prepare for my self-confidence to slide down the tubes.