Reading the scripture reminds me of the times I was always in the gym! (I've slacked off but don't judge me LOL) I love working out, love the way I felt afterwards when all the endorphins are firing away! Physically I felt great, mentally sharp, but what did it do for me spiritually? Nothing!
I don't care how toned, fit, skinny, etc.you are due to physical exercise, if your spiritual muscle is flabby you can't be a good servant of Jesus. You can't pass on the sound teachings that you were raised on. You must exercise daily in God! The message translation says "Stay at your post reading scripture, giving counsel, teaching." These things will build Godly character and spiritual muscle.
Now I'm not saying to go out there and cancel your gym membership, I'm simply saying you need a balance. And if your scale tips more to the side of a disciplined life and God, you will be more fit today and forever.
Now go out and show off your real muscles, the spiritual ones!