I had big hopes for this one, as it is a flat run, and had trained well.On the morning of the race, the temperatures started to climb, and climb. The run started well, lots of runners seemed to have a very relaxed approach - stopping for selfies, taking videos, and lots were walking right from the beginning.
At 7km, the first casualties went down. It was getting unbearably hot and there was no shade.
By 8k, there were more bodies at the wayside getting medical attention. I took a complete head stagger, and decided to abandon the PB idea & slow right down, and start speed walking. I had a flight home that evening, no way was I going to A&E instead!
It was a loooong walk. The heat was relentless at 25 degrees, and high humidity. As the race progressed, it was broken up by the sound of sirens from police and ambulances. The fire crews were out at a couple of points with hoses and sprinklers, which was so welcome, if short lived. I took salt tablets, a lot of water and electrolyte drinks.
About 2k from the finish, a water station was handing out water and oranges. The resultant mix of water and orange peel on the road turned it into a skating rink, and more runners went down.
Finished in 3 hours, the slowest time I have ever done, but never as glad to see a finish line. I had sunburn on spots where I missed putting on suncream, and my face had big streaks of salt from sweat. But its done! Afterwards i learned that an elite runner pulled out at mile 10 because of the heat and humidity.
Morale of the story - if conditions aren't right, change the plan. Getting to the finish becomes way more important than any PB
A soggy Half marathon tale 
Colchester Half
Inverness Half Marathon
The Juicyju Bath Half dissected 