Anaerobic threshold is not defined by a speed - it is defined by the mix of energy production systems your body is using. So continuing to do the intervals at the same effort level and heart rate, even though your pace slows, will provide the benefit you are looking for. How did you determine your threshold pace?

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Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
Instagram • Facebook

How were you measuring you threshold pace?

Threshold pace is about my 10k racing pace as a guide. My experience shows it's generally a good indicator of when my HR climbs up to anaerobic. When it's hot, of course, pace slows down while everything else is still working hard. I understand that "pace" is not the magic indicator, and pay closer attention to HR and perceived effort on my interval runs, but use my 10k racing pace as a general guideline and go from there.

So: as long as my PE and HR are appropriate for interval/anaerobic work, I'm on the beam - is that generally accurate?

I suppose 10k pace is not too bad an estimate.... though lactate testing is by far the best way to determine it. So normally holding 5x 6min all at the same pace should not be a problem at all right? Just something about the excessive heat that one day?

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Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
Instagram • Facebook

1 or 2 depending on what you are trying to accomplish in this session and where you are in your season.

If you have not done this set before, perhaps your calculation of threshold pace was a little off. Pace is usually a matter of trial and error and you may need to back off just a bit next time.
If you have done this workout before but now find yourself unable to keep pace on the last two, it might be time to bail, especially if you have a race coming up.
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Couple of recommendations. Always leave a little in the tank. Never completely exhaust yourself in a workout. Save it for racing. I favor some type of a way to regulate effort or pace when doing tempo type intervals. Otherwise people run them too hard or too easy (or both in the same session!). It is normal for heart rate to be a bit elevated if it is hot, especially if you are not heat acclimatized. So, letting your heart rate drift upwards a bit is OK as long as perceived exertion is normal. It is tricky business though. If you have heart rate and pace, you can generally sort it out. Otherwise, it is not going to matter too much if you just hold your normal pace or heart rate. You don't need a tremendously high level of precision with pacing tempo workouts. If you are somewhere in zone 4 or 10K race pace, it is good enough, again, as long as you leave a little in the tank.

Quick edit....I am generally in favor of completing the workout you scheduled, even if you are feeling a bit off. That way you don't start compromising too many of your workouts. Then you adjust your schedule after the workout to account for the fatigue if necessary. The exception would be if you are really, really trashed. In that case you probably ignored earlier warning signs and should have adjusted earlier.

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Simplify, Train, Live

Thanks for the great insights everybody. @rambler and @rowtotri, yes this is a manageable pace for me, if a touch hot. No pun intended as it was the heat that really snagged me. @Mike, good advice, especially for marathon training like I am.

I think I'll look at backing off my LT pace a shade and continue learning (hey, it's only been 10 years of doing triathlon) to deal with the heat. Appreciate the advice and support, thanks.

Use a heat adjusted pace according to the ambient temperature, Jack Daniels has an easy calculator. Should provide a good starting point and adjust as you go and how the pace feels and how your hr responds.