Being unhealthy and generally unfit is expensive. Living a truly sedentary lifestyle (one in which the word “exercise” is avoided at all cost and bursts of physical motion are vanishingly rare) is associated with all kinds of expensive illnesses and health risks. It literally costs you money to be lazy and out of shape.
But being fit and healthy is affordable by comparison. You can save yourself all kinds of money on healthcare costs and lifestyle expenses just by working your muscles periodically throughout the day. As far as frugality goes, physical fitness is an all-around genius tactic for saving.