Is it normal to beat obesity and the world's energy problems this way?
Forget the idea of putting overweight people on treadmills to generate energy. They won't do it, anyway. I've thought of a more direct solution. The human body converts food to energy (heat, electrical, etc.) What if we tapped that energy and ran all our devices off it? People always want more devices and use more power than they should so the impetus on eating would be to produce power that is then consumed. Everybody would be the right weight as all spare energy reserves would always be used. And we wouldn't need batteries or power stations.
Don't dismiss this until you've truly thought about it.