Grouse Grind in the summer

lWhen I was doing the grouse grind, I was in my zone 5 but my heart was beating fast and I was breathing heavily. I could feel my face being red and my legs were exhausted. I was really tired to do another step. My sister and I were going at a moderate pace and after the half way mark, hiking up became 100x more difficult. I couldn't go any more because I was exhausted, had no energy and my body wasn't used to doing this type of activity.
I think the body system was aerobic because my heart rate rose and it was an endurance activity.