Trenbolone, compression suits and a Bowflex will keep them fit until they reach Martian gravity.
No substitute for gravity........
Look at how many Bowflex's are for sale on Craig's List.........
Kelly's problems were due to a year of negative gravity and that's the major factor to overcome in extended space flight that would take us to the stars...........
Mars' gravity is only 38% that of earth so the human body could handle that for an extended period of time.......but there would be muscle degeneration that would have to be regained upon return to earth. And the older the astronaut, the harder for that to happen........