I train in the South Eastern part of the US. Training wise, it is a lot harder in the summer. In the winter time, I can go all day at a 7-8 minute pace without breaking a sweat. Now just getting 60 minutes easy is hard. In February I did a 10 miler, went through 10k in 36 minutes and finished in 58 minutes. Recently did a 10k, finished in 38 minutes. There is this one hilly route I run quite often. In the winter time I'll finish 10 miles easy in 70-75 minutes. Now in the summer time, the same route, I'm lucky to finish under 80 minutes. So yeah, it really sucks. Though as soon as it cools down, my easy 8:00-8:30 minute miles magic get faster by a minute or more. Just a matter of putting my head down and grinding it out and reminding myself I haven't lost fitness. It's just hot and humid. Also, training with the Sun down helps a lot. I see why Mark Nenow ran at night.