I did the rim to rim hike over 4 days, 3 nights. We hiked in from the North Rim.
I was not in nearly as good of shape as you were, but in probably the best shape of my life.
It was an amazing trip, but the hike out was no fucking joke. And you did it in half the time I did. Hard to imagine.
I distinctly remember seeing runners who would run by with no camping gear, meaning they were presumably doing the entire trail in one day. I don’t understand how that’s possible.