I have treated a horse for colic with the history of him being fine out on a trail and then he got sick after drinking cold water. I think that the horse was EXTREMELY overheated when they gave him the water. You really need to be careful not to work your horse to the point of exhaustion in the extreme heat. If a horse is in pretty good condition, six miles should not be that hard for him. I always try to walk the final mile home so that they can cool down. The colicy horse that I treated did fine. He got better fast, and had no long term problems from it. In the future, the horse owner gave the horse water before riding, and tried to cool him off before giving him extremely cold water.
As far as me, I am going to give out WAY before my horse gets extremely overheated. I know that a lot of you on this forum are a lot younger and more athletic than I am. Oh to be 20 again........