Interesting study but it won't impact my training plan too much, as I always thought (and felt) that it's the mix that matters: Zone 2 and HIIT, cardio and strength and mobility, workout and relaxation, cold and heat, breathing and breathholds, etc. Regarding cardio: If I have only little time I do HIIT (e.g. Tabata) as it's most time efficient. If I have much time and feel like doing a long run, I run in Zone 2 – simply 'cause I cannot run for hours in Zone 4 or 5 ;) But at least I will have less guilty conscience now, each time my pulse exceeds the zone 2 limit :)

Two additional remarks:
1) All the experienced running coaches I know recommend ambitious runners who have been running at their "comfortable pace" all the time and are surprised that they are not getting any faster: Run slower! And that always leads to progress! Second recommendation: Then run faster ;)

2) Everyone talks about Zone 2, but I believe there are many different definitions of it and how to calculate/derive it - just by comparing the heart rate suggestions of the various fitness devices and apps ... I stick to the zone just below the first lactate threshold, so I'll be doing another lactate test soon.

Thanks for the tip! But I don't think this is really new ‘in the context of lower training volumes’. It's about the perfect training combination (which promotes performance improvements and muscle growth, but also creates a strong foundation in connection with optimal recovery and efficient metabolism). A lot of training in zone 2 (and even zones 1 and 0) while increasing the training load is really helpful and the way to go. But yes, if you don't have enough time (e.g. only 3 hours per week), it can make sense to do harder treshold or interval training instead of zone2. However, what I often observe (among non-professionals, <= 10 hours of training per week) is that 90% of people then train too hard and fail to build a solid foundation – which often leads to injuries, illnesses and missed training sessions. So should we still focus on Zone 2 and lower intensity training! YESS!! But not as replacement for VO2Max and treshhold trianing but in combination with increasing the training load.