I was a borderline elite level distance runner, D1 college (if that's worth anything to you), etc. Cardio didn't seem to do anything for my stutter. What does help and did help is the confidence from looking and feeling good. I got into mixed martial arts later and cut down on pure distance to focus on HIIT and strength training. Same deal - the physical shape didn't matter so much as the way I carried myself if I was staying healthy, injury free, and doing well competitively. If something happened to affect my confidence, my stutter did get worse even though my physical shape was unchanged.