Listening to your body is the best way to decide if an exercise pace is right for you. If you do too much, you'll get tired too quickly or get hurt. If you do too little, you won't get the health rewards you want.
During or after exercise, you should not feel sick, dizzy, or extremely tired. After you're done, you should feel normal again in about 10 minutes. If you're very tired or very sore the next day, you may have exercised too hard.