Unless we overeat, we shouldn't. And that's a different issue imo. We can just as easily chop our own wood or hand wash our own laundry to burn those excess calories. Assuming a healthy diet, and no other possible source of exercise and utility,
AFAIK 5 miles per day of cycling at 15mph is all that's needed to *keep up health. Anything else is just incurring the extremely high cost and inefficiency of our food supply.
*This was from a European study of some sort, pester me if you want the details. I'm pretty sure I can search for it again.
I couldn't eat junk food when I biked as little as 75-90miles/week or I'd I'd bonk while riding, and the same went for smoking/alky. Cycling forced me to eat fairly healthy.