[jasonhansel]

In my experience, when you go on very long (10-20mi) walks in hot weather, you need a backpack even more, since you have to carry around large amounts of water (either in water bottles or in one of those hydration pack things).

Biking is different, of course--you're not outside for as long (at least for a given number of miles), and you can store your water bottles on the bike itself.

[dredmorbius]

Hydration bags tend to be slightly less problematic as many incorporate some amount of airflow-management and moisture-wicking, and the water itself tends to be somewhat cooler, at least initially.