I took Rescue Diver twice (literally washed out the first attempt). The first time was when I started PSD diving and the instructor was a former Navy diver. I didn't last long. I was out of my element, my wet suit didn't fit right, not enough experience, my comfort zone was much too small, not in good enough shape and the instructor was VERY demanding and intense. I became what we were learning about; a stressed diver.
I went down this list and took care of each of these problems that added to my stress. Two years later, I took this course again and cruised thru it.
Intensity and exertion level varies greatly from instructor to instructor. However, diver "comfort" is as important as fitness. Get real comfortable in the water with your skills and gear.