Only for Old Women

But have nothing to do with worldly fables fit only for old women. On the other hand, discipline yourself for the purpose of godliness; for bodily discipline is only of little profit, but godliness is profitable for all things, since it holds promise for the present life and also for the life to come. 1Timothy 4:7-8

First, I doubt Paul was anti-physical fitness. Rather he felt spiritual fitness was far more important. That is because godliness affects every part of the believer's life. The amount of sleep we get, what we eat, our careers, hobbies, and ministries are all subjected to how we feel about God and the things of God.

Additionally, knowing how life ends – with us victoriously and gloriously in eternity with God – gives us an uncommon hope for the future; those who do not know Him do not share that hope and, indeed, have little hope or, worse, rely on a false hope.

In order to help us focus on the godly life, Paul makes the anti-worldly fables statement. His reference could be to Jewish (the traditions of the elders), or Gnostic (about God, angels, and creation), or any number of false doctrines afloat at the time (commands to abstain from meat or marriage). World fables exist today in Open Theism, the emerging church, and hyper-Calvinism to name a few.

Paul urges the Christian to read, study, and apply the Bible's truth, not what he might think the Bible says.