Weakened immune system. Don't try to push through that exercise funk, Talbott warns, "or you'll keep sliding down—to a weakened immune system, inflammation, and outright injury." Not a good thing. Prolonged overtraining can take weeks, even months, to recover from, and can put your health at risk. Chronic inflammation, for example, has been linked to diabetes, heart disease and cancer. Bottom line: Nurture your body and give it a much-deserved break when it needs to rest after that tough workout.