1) Give your pre-ride nutrition more time to digest and get into your system.
2) What have your rides been like up to this one? I’d you don’t normally do a fair amount of climbing maybe this was just too much right now. Build up to it next time to the point where you’ve done several rides of 70-80% distance and elevation as the one you want to do. Plus some of the same or longer distance but flatter where you push it hard. Then you’ll be more ready for the longer climbier ride.
Was going to guess the heat got to you but then you said the temps weren’t that bad