mrog, I too commute in Phx, but only 9mi, so for my afternoon commute I start very well hydrated and maybe have a few water sips on the way home to wet my mouth and then drink at home. But that is only 30min exposure, which is about my limit in the full sun mid-summer (115+). Even then it is possible to loose a couple water pounds. For 35mi if you are commuting in afternoon daylight you will need lots of water and maybe even a cool down stop or two on they way - as JB said that in the hot dry you are limited by what your body can actually use - there is only so fast one can sweat.