It's amazing what our bodies can do and grow from. This is my fourth time doing this and by then end I'm pretty beat up but also stronger and with much higher endurance like you said
What I noticed is that even if I’m dragging to start. My body has become accustomed to the work and another gear just kicks in. When I am done, I usually think, I don’t know how I just did that. I didn’t think I had that much energy, but I guess I did.