Wildbill - been out every day this week. Yesterday I averaged 15.4mph for 90 minutes (warmed up, picked up speed and just stayed there until I didn't feel comfortable). The heat was much more subdued, and I had a couple granola bars with breakfast. It was much harder than I've gone before, but no feelings of sickness like before (just a feeling of "depletion"). I added another water bottle to my bike and made sure to take a few sips every 5 minutes or so.

Thanks for the intelligent responses, folks.