Yesterday, I did my training on the tracks: 4 x 400 m (100 m walking), 200m walking and then 2x800m (100 m walking). I noticed that the first 4 times 400 m, my heart rate was between 150 and 160 bpm. The 2 times 800 m gave a heartrate of ~180 bpm, which seems almost the same as my cadans. I'm 45, so the change that my heartrate is this high for these 2 series is not very likely and the average pace was muck lower! Could this assumption be true?