I see a lot of good ideas from other posters here but two things missing... A) Start earlier, I am out in Arizona, so I am familiar with heat issues, I try to get out at 5am. B. Besides drinking before rollout, put yourself on a hydration schedule). Using your cycle computer elapsed time, to keep you on schedule, drink at least a bottle an hour. I have found out that if I fall behind schedule, sometimes it an be difficult to catch up. One time I did the Davis Double (century), ate too much lunch (another thing to watch out for), fell behind a hydration schedule and like you, I was spent after 175 miles.