I get toxic-feeling and irresistibly drowsy after multi-hour efforts on the road, but maybe I've never beat on myself bad enough in the gym to experience it. It is said that the immune mediators released during prolonged, hard, exercise have considerable overlap with the ones circulating in systemic illness, like flu. Somehow, they feel a lot better when you do it to yourself, instead of getting mauled by a virus. It's also interesting that that those same mediators are considered markers for badness in people with obesity, diabetes, heart failure, etc., but they just make us Übermenschen stronger.