2/ Thinking of the possible reasons, one could be the high humidity in the past two days. On Saturday we started out with 80+% humidity, and on Sunday it was 91%. The temperature was mild (~19 Celsius), but tiredness more quickly set in for me with no particular muscle soreness etc. With the 5K runs, my heart rate was routinely in Zone 5 (160+BPM), meaning a bit thicker air might have done me in.