Found out the hard way that pre-hydration is key for me. I take a ton of fluids with me as well. Warm up slowly. Use a HR monitor if you have one, easier for me personally to gauge my effort when it's hot. If it's hot and windy, like I ran into yesterday, my rides will be shorter and closer to home.
Heat and wind drive the HR way up for me when I climb anything.
Just take it easy baby!