"I don't think I have the mileage on me to really complain of any injuries yet"

Shannon Miller is a seven-time Olympic medalist in gymnastics, and this quote is most likely describing the physical toll that her sport has handled her body. She is implying that she has not been contending long enough to have any severe injuries, which she is still relatively healthy. This is likely a referral to the truth that numerous gymnasts experience persistent injuries due to the intense physical demands of the sport. Miller is likely revealing her appreciation for her existing physical condition, and her gratitude for the fact that she has actually not yet experienced any major injuries. Her quote is a suggestion to other professional athletes to look after their bodies and to be conscious of the physical needs of their sport.