I did 50 miles yesterday with temps in the 90's. I started out with two bottles of water, stopped at Walgreens halfway through and bought a bottle of water and a bottle of gatorade. I mixed them 50/50 in each bottle as I find straight gatorade is tough to take. It worked out well. In the past I've bought gatorade powder and mixed one weak bottle and kept my other bottle straight water (for when you need to dump some over your head).