All good advice, and may I add one more...
your feet will swell up A LOT after 2 or 3 hours riding on a really hot day, so make sure your cycling shoes are comfortable, and with some wriggle room. Do not begin your ride with them cinched up tight. Take the time to loosen the straps at a rest stop BEFORE it becomes a comfort issue.
Oh, and what Tim said about taking some salts capsules - wouldn't be without them. I've helped numerous mates who began getting severe leg cramps after 60 or 70km.