A hard day of running should be followed by a day of active resting, which could include an easy jog. If you don't allow recovery time, you'll soon see a decrease in your performance, experts say.
Listen to your body. If you feel more exhausted than energised despite your best exercise efforts, it's probably time to take things more slowly. Also watch out for signs of overexertion, such as an increase in resting heart rate, muscle or joint pain, a bout of flu, or a decrease in appetite.