The commandment “Thou shalt not kill” comes into play here. While it wouldn’t bring about an imminent death if one were not to hit the gym, long term, it may indeed have consequences.
We are required to “take care of the temple.” St. Paul writes that we should “glorify God with our bodies.”
A second note: The Greeks believed in dualism, meaning that the body was bad and corrupt and that the spirit was all that mattered. That gave rise to many not caring about their health, mostly also because they expected the world to end relatively soon.
But “we do not know the day nor the hour.” God wants us to be the best version of ourselves. So while we don’t have to have ripped abs, or buns of steel, we should take care of the body as best we can, by eating healthy food and getting regular exercise and sleep.