What others said, but adding my 2 cents.

It's not the end of the world to get a little dehydrated barring any unusual medical conditions. In fact, I think a person should learn what it feels like in a safe environment, so they don't panic if they find themselves in that situation on the trail.

One thing I've learned road walking in temperatures over 100 degrees is it's more important to conserve sweat than to drink water. When it's 100 degrees, the temperature above the pavement is about 140 degrees, so it can get toasty.

To conserve sweat in these conditions, I get off the road every 10 or 15 minutes and find a shady spot for a couple minutes until I cool down. Usually, I can make it at least 10 miles in these conditions without drinking water. Less in the beginning of the year when I'm not acclimated.

Same idea with backpacking. Don't charge uphill on a hot day and end up soaking wet.