How is this for cardio?

%80 of max heart rate. I’m 41 years old. 6’1 220. I do that three times a week. Other two days I do 20-24 intervals of 30 second run with 30 second rest. I started doing the 30/15 last week and it’s tough, but a good quick, high intensity workout.

And for those of you wondering I just hop off the treadmill (straddle the treadmill) instead of stopping the treadmill during rest period

Resting and Max HR aren't direct relations to cardio fitness necessarily. For instance at 32 years old, I can get in low 40's for resting HR when in shape. My Max HR is very high compared to most elite endurance athletes. I'm pretty slow when it comes to distance running for reference. You would expect from my resting HR that I'd be pretty efficient, but not totally the case.

I think the pace would likely need to be picked up a little bit for an interval that short. Totally depending on your ability though. For instance, an up-tempo endurance run would be 3 miles at an 8:30 pace (again, I'm slow). A high intensity run would be 7 and change pace for a mile. Quarter mile repeates would shoot for 6:30 pace. And those quarter mile repeats are well over a minute in total time. So 30 seconds should just be all out. 18 of them would hurt pretty bad from 13 and up.