I agree on the perspective that the muscular system is a secretory organ that constantly secrete molecules to the rest of the body. Muscles are highly metabolic, easily targeted, very responsive to interventions and highly adaptable to the activity they perform.
With exercise intervention we can influence what kind of substances the muscles secrete to the rest of the body. One can say that in, a very imprecise statement, that when exercising and building healthy muscles you reap benefits of whole-body rejuvenation, in a way that is similar to what researchers try to do with parabiosis, plasma exchange, plasma dilution etc. But the thing with exercise is that we know it works and that it improves health span, and it is available to almost all people.