10 years ago I started doing HIIT via the 7-minute workout while initially out of shape. The first month was horrible, then my body substantially improved, I stopped being lactose intolerant (could suddenly drink a gallon of milk a day without any issues), then I could stack 2 HIITs and later 3 HIITs in a row, then I could push more and more reps in each 30 second interval, achieving like 4-6x improvement compared to when I started and my fitness exploded.