Working out means keeping your physique in good condition and above all, taking care of your health, provided you don't go to extremes. Often people confuse pushing their limits and going too far in the process of working out.
Rhabdomyolysis, the overexertion syndrome
Overtraining when you're not used to the pace can cause rhabdomyolysis. This syndrome destroys muscle fibres and releases a protein called myoglobin (a protein responsible for the red colour in certain meat).
Myoglobin is not toxic, but its release is dangerous and can cause major problems in the kidneys and even the heart. Thus, if after a workout you have muscle pains (not to be confused with body aches) for a long time afterwards, and your urine is particularly dark, consult a doctor immediately.