> A stationary bike at a steady 20mph pace is about 500 calories in 30 minutes. That's really significant. An hour will erase about a third of a normal person's diet.

That assumes a “normal person” will not compensate for the effort with a snack.

A “reward” pint of ice cream will re-add more calories than the hour of cycling. A “standard serving” will nearly match your half hour.

And that’s an hour at 20mph, which really isn’t in any way the norm.