What is the grey zone in running?
The benefits and detriments of running in the grey zone
How to use the talk test 
When to use zone 3 training
Should you do all of your long runs at marathon pace?
What is polarized training (hard days hard, easy days easy)
Marathon pace/zone 3 workouts
Zone 2 vs zone running when you are a beginner runner
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2: Zone 2 training and heart rate
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References: 
PMID: 33344993
PMID: 36900796
PMID: 35661059
PMID: 26578968
Skiba, Scientific Training For Endurance Athletes
https://www.frontiersin.org/articles/10.3389/fspor.2019.00070/full
Further reading: 
https://lauranorrisrunning.com/what-is-zone-2-training/
https://www.runtothefinish.com/zone-2-heart-rate-training/
https://www.runtothefinish.com/running-heart-rate/
https://lauranorrisrunning.com/training-intensity-distribution-for-runners/