Agreed. I'm a skinny but relatively fit person myself too and actually experienced what I think was a bad case of dehydration last night. I live right near the base of Slate Canyon in Provo and wanted to go from Slate Canyon to Y Mountain trailhead, head up Slide Canyon, then loop over and out of Slate Canyon. So I ventured out yesterday afternoon about 4 PM thinking I could do it in 4 hours and be back before sundown. I took a pack with two liters of water but failed to remember a bag of trail mix I meant to take as well. I followed the BST from Slate Canyon trailhead to Y Mtn, then up the Y (by far the most strenuous stretch) and up into Slide Canyon. Never being up that far, I was going just off a map. I got up to a meadow where the trail was covered up by dense wildflowers and so on. By then it had already been almost 3 hours and rather than push further up my intended route, I turned back and came down the Y trail. Once I got to the bottom of the Y, I really started to feel exhausted.
I kept sipping from my water, but ended up running out on my way back along the BST to Slate Canyon. I started to feel some nausea, more tired, and started having dry heaves. Once I reached the Slate Canyon Trailhead, even though I was less than a mile from home, I was feeling so miserable that I resorted to calling my wife to come pick me up. Once home, it took me over an hour to feel recovered and get my appetite back after laying down and sipping away a liter of water, a can of soda, and a drink mix. In hindsight, I realized I had skipped out on lunch earlier in the day because I was busy taking care of some other things around the house. I've done tons of day hikes in my lifetime (I was even down in the desert at Calf Creek this last Saturday carrying a child on my back both ways) and never experienced the level of fatigue, exhaustion, dehydration and the scary feeling that comes with it as I did last night. Luckily for me, the sun was blocked by some nice clouds out west while I was on my way back along the BST, otherwise I or someone else may have been calling 911.
I've learned my lesson: Never begin a hike on an empty stomach, don't forget some snacks for the trail, and always take more water then you think you'll need. It's not that I set out that way intentionally or that I was ignorant, but I now know from first-hand experience the importance of being more prepared, no matter your fitness level.