Best metric to use for long runs in summer heat w/ humidity

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    This is my first post and I am training for NYC Marathon and am in week 3 of Level 2 Marathon Plan.

    I have done the level 3 plan in 2020 pre covid and am using all three, pace, HR and power durning this cycle. For my foundation runs I follow HR as per Matt’s recommendation in the book and follow pace for tempo runs and power for hill workouts.

    Question, for long runs in zone 2 I tried to follow HR only to find myself drifting after 60min in to zone x and higher. I tried this past weekend to split the 18k into two 9k intervals with the first by HR and the second by pace. The result, first 8.5k was 6:27/km w/ HR avg 166 and the second 8.5k 6:10/km w/ HR avg 180 with the last km at 185bpm. the temp was 23 w/ 77% humidity.

    What’s the best way to execute the long runs in the summer? Thanks

    I did a 20min TT before started the plan to have accurate zones.

    Distance – 4.30km
    Pace – 4:39
    Power – 339 W
    Last 15min avg HR – 184bpm

    I’m 45 and 5’10, 205lbs

    My marathon PB is 3:28 from 2019 Chicago

    David Warden


    There is certainly a disconnect between your threshold test and long run, almost certainly has to do with that humidity. For example, you averaged 184bpm for a 4:39 pace in your threshold test, but at the end of a long run you averaged 180bpm for a 6:10 pace (that’s a 2% difference in HR for a 25% difference in pace).

    Cardiac drift is normal, even for the fittest athlete in perfect conditions. That level of cardiac drift is high, again, probably the humidity, but HR can be influenced by many other factors as well.

    In cases where you are trying to use Pace as a primary and HR as a secondary, the “rule” is not cross more than one HR zone. So, if you are doing a Zone 2 pace run, and HR drifts into Zone X, that’s OK. But, if your Zone 2 Pace run drifts into Zone 3 HR, then slow down.


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