Despite my recent complaints about heat exhaustion, I try to get out for at least 5 hours when we get into triple digits. Having said that, humidity is low here in the summer so 100 degrees in OR isn't nearly as bad as the same temp in other areas.

You have to drink insane amounts of water. On one of my training hills, I once went through one bottle every 2.5 miles (I hid gallon jugs of water along my route). I also find I have to ease up on the pace a little bit.