Double day…
Couldn’t quite get 4x6 done for a second time - only 4 mins in the final interval. Plenty of time breathing like a fish - actually got a bit panicked at one point and had to turn off the Vacmaster that blows into my face, as I felt like I couldn’t breathe. Good times 
Big drop off in time spent around 93-94% HR max - no doubt a symptom of a 2nd workout 7.25 hours after the 1st one.