I did a 12 mile bike ride today at an average of 19 mph. I then did a 2.5 mile run at an average pace of 9:20 per mile. According to the Garmin app, during the run, I was in zone five 67% of the time, zone four 27% of the time and zone three 4% of the time. I was never in zone 1 or 2 (not sure exactly why the above percentages only add up to 98%; perhaps due to rounding?). I used a Forerunner 735 XT for the run, linked to a Garmin chest-strap heart monitor.
According to the Garmin app, my training effect was 3.8 for the run. This does not make sense to me, given that I was in zone four or five 94% of the time during the run. I would add that I am 65 years old and have correctly set my max heart rate as 156.
Any thoughts on what the problem might be (if there is a problem at all)? I recognize that it might have something to do with the fact that the run was not a particularly long workout, only lasting 23:23. Thanks in advance for any help provided!