I live in southern cali, and it gets very very hot here. I have gotten so overheated on the trail I've been physically ill and nearly passed out. I agree early mornings are better then evenings (both are better then the heat of the day), but if you know you are heading out when it's hot keep your neck cool. Keeping all those main veins cool will make a big difference. I take a bandana, soak it in water and losely tie it around my neck before heading out. Between that and resting frequently and drinking lots of water you should be ok. I have asthma too but it's such a dry heat here it's hasn't been too much of a problem for me. Good luck.