WOW,
I took a CPR/EAD certificate renewal class last month. This one is really good in that we got a lot of practice in one, two, and three-person CPR (three being the best as one person does the chest compressions, one breathes, and one rests and times.)
I lift weights and get a couple of hours of decent cardio a week, and until recent sciatica ran a bit. And at TWO minutes of 30/2 (30 seconds of chest compressions, two breaths, repeat) I was wiped. Doing them properly is 75% the effort of pushups. I can’t imagine someone being able to do 30/2 for more than 5-6 minutes unless they are in ironman level shape.
One of the mistakes people make doing CPR is to try to use their elbows to do the compressions and keep their backs more or less rigid. That is going to wipe you out quickly.
Keep your elbows locked out straight and use the weight of your upper body to do the compression and then your lower back to raise your back straight. You would be surprised how long you can go like that.