I felt the same thing when I did yoga on my own in my house. I always had yoga classes in school which was less than half an hour and my stutter was indeed less during my school time. When I started to do yoga in my house, especially with more meditation, I completely stopped stuttering for close to six months before I started to stutter again when I found no time for yoga. And regarding cardio, I always noticed I spoke very well whenever I played football(soccer) and I do continue to speak very well for a few hours after I play.