HIIT/SIT or MICT/LSD? Simple answer, the two

https://www.frontiersin.org/articles/10.3389/fphys.2021.660818/full 

My experience is that I cannot do HIIT all the time for instance running workouts. Spontaneously, in a week I mixed LSD and a series of 10x400 m or short hill climb workouts.
I have more difficulties doing that in swimming and biking.