Me and two friends, also in our early 20s, found this out as well. There are plenty of signs telling you not to hike down and back in a day, but we (mostly me) were young and stupid. I remember passing the signs thinking, meh, that’s just for old people. I also remember a sign suggesting at least a gallon of water per person, per day, looking down at the 20 ounce squeeze bottle we had between the three of us thinking, that’s just for thirsty people.
It took us a little over an hour going down. My one friend, who happens to have a heart condition (leaky valve), insisted we turn back. I wanted to press on to the Colorado river, which was another 3 miles away. We had four hours more daylight, after all. But thankfully he dissuaded me, and we started back up.
In contrast to the steep descent, going uphill was sloooooowwww. And the sun was baking us. Almost immediately we were running out of water. Feeling sheepish for egging my friends on most of the hike, I stopped drinking to save water for my friend with the heart condition. But that didn’t make him hike any faster. He was able to walk only 50 feet or so before becoming extremely winded and needing to rest.
After a couple hours, I contemplated stopping with my winded buddy while the third friend went for help. Truthfully, I was feeling pretty sh!tty as well, likely from dehydration. But we pressed on, eventually making it back to the rim just as night fell.
As soon as we got back to our campsite, I downed a two liter bottle of Dr. Pepper. While it tasted great, the body is not designed to drink so much carbonation and sugar so quickly. Unlike the hike, it came up much faster than it went down.
While we all survived, the moral of the story is, pay attention to the signs, and don’t attempt to hike to the bottom and back in a day.