I think drinking too much is a habit too. I would think you should look at other causes of repeated headaches and a 10 mile ride would not be a cause to need 32 ounces of water and salty food. I ride in Ut and it is hot in summer but I know I drink too much when I need to pee too much (like on a 2 hour ride). Dehydration is a term bandied around very loosely. I do tweak my own electrolytes depending on conditions and ride times, but for a nice rec ride of low mileage, it should not be an issue. I don't race or anything but bigger climbs and really hot days will need different approaches. I would not listen to a doc who tells me not to drink at night either, I can get up and pee and go to sleep. When I am out on my bike with no loo in sight, I am not so keen on peeing.