Cosgrove recommends that if you have between 3-5 hrs a week to train that beyond the 3 hrs you devote to weight training, you should add in the next 2 hrs with HIIT.
after an intense weight training day, how much juice can one have for HIIT? maybe 10-15 mins worth? guess as you adapt you could slowly increase a little
now that leaves you with 4 non-weight training days to include the other 1-1.5 hrs.
after you do say 20 mins on each of those days, would you then include regular interval training? seems to me, after a session of 20 mins of HIIT, you would be gassed and have nothing left for interval training… maybe rest a few minutes then finish with intervals?
sorry if this sounds a little confusing.