I ran around 11.5k yesterday evening. It was still quite warm out, and though I felt like I was running just a tad faster than easy pace, my time on the first km didn't back that up. Maybe the heat, maybe the long day at work. I'll blame the warmth, 'cause god knows living here you don't get acclimatized to the heat.
Don't misconstrue my comment, I'd love to see more of this type of weather and get the chance to run in it more often.