Keegan talks about some of these issues in The Face of Battle. Contrary to Hollywood and fantasy books, armor wasn't worn all day, but it was donned just prior to battle. Of course there were baggage and supply trains with water to keep soldiers hydrated. Check out this post from /u/abt137 2 years ago: https://www.reddit.com/r/AskHistorians/comments/2r9orp/water_supply_in_an_ancient_battle/

However, sometimes I think the phrase, "died in the heat of battle" may have been much more literal...