Janie, you have to work up to the heat factor, especially when cycling off-road. I'm blessed with beautiful summer time temps usually in the mid-80's and plenty of tree shade, but usually several times a year I may be riding out west where temps reach into the 90's and 100's (think Fruita/Moab).
Your body will acclimate to the heat eventually, and it helps enormously if you hydrate and feed it right before you ride. Wear only fabrics that are polyester/lycra with lots of breath room. In your helmet, you can wear a sweat dam, what I do is take a bandana and fold it into a rectangle and place it in my helmet for added *shade* and sweat wicking. Wear short-fingered gloves and polyester socks. I try to ride after the heat of the day, but that's not always possible. Before I ride, I usually have a bowl of oatmeal, like Quaker Oats and then 16 oz of water to pre-hydrate. Ya...I know the potty factor, but on hot days, you'd be surprised.
I'm not sure if you have a camelbak, but I highly recommend one. You can fill the bladder half with ice, then water. When you sweat this hard, you are loosing minerals which will give you that whoosy feeling after a few hours. The ice will melt, but you'd be surprised how cold it stays in a camelbak. Sometimes I use Hammer *Heed* in my camelbak, but 85 percent of the time I put about 40 drops of ConcernTrace Minerals in the water, depending on how many ounces. I never cramp after long sweaty rides if I follow my rules. Hammer also makes great gels for quick pick-up, and the schnitz is a Payday Bar, and not the generic peanut roll, but the PayDay Bar itself is probably the best thing I've found to put in a pack. They don't melt or gross out, they have salt, sugar and peanuts (protein). I can't stress enough though Janie, you have to take it slow. Your body will get used to mountainbiking and the added heat, just take 30 minutes stints until you build up to it.
