One day, about 12 miles into a run, heart rate (HR) increased,
respiration rate increased, and ability to do work/run decreased dramatically. Next day I felt fine. A few weeks later, same thing happened about 16 miles into a run. Tested nutrition, hydration, electrolytes, and combinations of three with no change. Symptoms would persist for up to several weeks at times, or be gone the next day. Symptoms happened 3-27 miles into my runs. Last March during 50k race, happened again at mile 15.