Would you go out for a leisurely drive in the middle of a snow storm? I'm sure the short answer is no, as you wouldn't take on the weather unless you really had to. With this in mind, it always baffles me why people start running when the sun is shining.
I live near several parks and you can guarantee that when the sun is out on the hottest days of the year, you'll come across someone looking really worse for wear who's cleary just started running that day. Heat is a great way to push yourself if you're in condition, but it's not a way to start out.
I shouldn't have to tell you this, but you'll soon be dehydrated and it goes without saying that you probably won't want to be out there the next day.
Do it right, be there for the the whole race and don't get taken down at the start.
Consider starting out when it's cooler and be in shape to make the most of those beautiful summer evenings. I'm sorry if this seems obvious, but the amount of people out there making this mistake suggests it's worth posting this.