I'm not new to it, have been lifting weights since I was 30, so well over a decade. I've only been doing cardio heavily for the last year or two, after an injury put me off weights for a while.
Also to be fair, I call it HIIT, but I'm just playing really hard DDR songs for an hour. The dips and spikes are the cooldown between picking songs. 😂
I've found maybe I'm hitting my cap currently. Over time I've been doing harder and harder tracks, and over time I get better, my HR goes lower during workouts, and then I start pushing harder tracks again, gets easier over time, lower heart rate, repeat.
Also not sure if it matters, but it feels like if my heart rate is staying under 160, it's completely unchallenging, like I'm not making an effort.