Any muscle you can build( which will be tough at 58......sorry buddy ) will be acquired according to demands imposed placed upon them greater than they are at present....thus the need for adaptation. Once those demands diminish due to going back to bodyweight exercises the need will no longer be there for such muscular size and they will slowly revert back to what is needed to perform your present workout.
Regarding the routine it is how you perform the exercises that is most important. You can pick a routine consisting of some awesome exercises but if you perform them poorly then your gains will be nil and the chance of injury will be high.
Have a read of this article as I think it clearly explains how to lift weights for optimal gains.....Yeah you would think this is obvious but from what I see in the gym it clearly isn't !
How To Correctly Use A Barbell, And How NOT To | High Intensity Training by Drew Baye