I’ve done this hike; to the very bottom campground, over night, and then hike back up the next day.
I was about 25, then (2011 ish) . I did nothing but field work (hike through the bush - no trails, dig when you get there) all summer, for 300 hrs a month. As a result, I was really used to walking for a very long time, under shitty conditions. Another co-worker came with me (he was into trail running/marathons). We went in May, started our hike down at 4 am (to beat the heat) and hiked back up about the same time the following day.
Let me tell you, this is on the list of top 5 hardest things I’ve ever done. Maybe even in the top 3.
the last quarter of the hike to the bottom is very challenging, because you get into some sandy/dune parts, and they just take whatever you have left out of you.
Hiking out, the bottom half (to Havasupai Gardens) isn’t bad, but then it’s swtichback city, population: you. I started off doing 5 switch backs at a time, but I wasn’t even to 3-mile house by the time I was dropping down to 1, or maybe 1/2 a switch back at a time, and then needing a break.
I can’t imagine being 40-50 and doing this in the heat we’re experiencing now; it was hot as balls when we did it in May.