Endurance athlete and unexplained SOB I think of a possible pulmonary embolism. I'm not nearly as fast as you but other than the muscle issues that was me 6 years ago. Could not get a full breath all the time. Felt like aomeone was squeezing the lungs. Went to 3 specialists over 6 weeks including a pulmonologist who all failed to dx it. Have them run a d dimer blood test as an initial screen. But I would not fuck around with SOB without a known cause.