Hey morrigu I'm crap in the heat too :(
Had a really terrible interval run on the treadmill at the gym last week when it was really hot in there - made it to 3k then left the gym and did the final 2k outside)
Realised a few hours later that I went at 11AM and all I'd had to drink all day was half a cup of tea at 8AM.
Took a 500ml bottle of water to bed with me the night before my next run and drank it before I got out of bed, about an hour before I ran and the difference was amazing! 10k outdoors no problems.
I'm crap at remembering to drink plenty before I go so apologies if I'm preaching to the converted but it made a real difference for me. I'm running in the morning and already got a bottle of water next to the bed!