That was just an example but if you think a 1:50 per 400 m is easy pace for most Ironman triathletes then why are so many run times over 3:12 marathon pace???
I am pretty sure at a random north American triathlon 90 % of the field could not complete 6 x 1 mile at 7:20 leave on o 9:20 without lactate exciding training goals.
You should rest 2 min btw a hard set to recovery to preform again. Or else you will not have enough recovery to do more intervals at the proper pace over and over again without a large build up of fatigue.