I have read a lot about people struggling with the heat at the beginning of the Pacific Crest Trail. I wish there was a good way for people who don't live in Southern California to train for it.
I have learned a lot over the years dayhiking in Southern California. Learning about my body and water has been huge.
I've been dehydrated a million times. It really isn't that big of a deal to get a little dehydrated. You can get over it easily by drinking water.
The real problem is when you find yourself drinking a lot but you are still dehydrated. No matter how much you drink you can never feel your thirst being quenched. Then you start to feel really horrible. Weak, queasy, thirsty and generally not good.
When this happens you need to lay off the sugary stuff. Stop eating energy bars and drinking sweet drinks, and stop drinking the plain water, too. It's really hard but if you are having this unquenchable thirst and are feeling worse the more you drink, you have to force yourself to stop drinking and have something salty. You might have to cook a little soup or maybe you'll have some Pringles or something extremely salty in your pack. Not Gatorade! Real salt. A diet lemonade with a big pinch of lite salt will do, but no sugar.
After you get enough salt in you, you will be able to drink water again and be able to quench your thirst. You will start to feel better, but it might take a little while.
I have read so many trail journals of people starting out and I can tell by reading them that this is happening to them. Living here in Southern California I have had this happen to me so many times that I can recognize it and fix it before it spirals. I had it happen to me coming in to Walker Pass. I recognized it, forced myself not to drink anything until I could make soup at the campground and get some cold water out of the trough. It was really hard to do, but I lived. At least it was all downhill.
I have a crazy boyfriend who loves to wake up on 100 degree days and he'll say to himself, hey I think I'll go hike Hurricane Deck (the hottest, remotest most awful place we have here where the sun bakes the rocks to the consistency of broken china dishes and there's no water anywhere). He went out there once on an overnighter in 100 degree temperatures with only water and energy bars and loppers to cut his way through the bushes. I decided to drive up to meet him on his way out at the trailhead with cold drinks and salty stuff. Here he comes barely alive stumbling down the trail. He nearly cried when he saw all that stuff. He had almost killed himself by not having anything salty and he wasn't sure he was going to make it out of there. He's nuts, but together we've learned a lot.