I just rode my 100 mile training ride yesterday.
Thinks I find mandatory riding in Texas midday typically are my camelbak (2liter) plus 2 1l bottles of gatorade/water mixture (60/40). I wear kit myself, and I can say that the coolmax-grade jersey helps keep me cooler than a t-shirt would. I bring along lots of vegan-grade organic bars, gensoy bars, etc - total of around 1,000 calories, including a bagel, a pbj and a banana on my weekly centuries nowadays. Originally I brought along just 2 gels and 2 powerbars and found that a. they were just too damn sweet and b. it wasn't enough for the 3k plus calorie deficit from the ride.
Try to set up training rides near bathrooms, water supplies (gas stations, etc) and don't push yourself, let the distance come first, then the speed. My avg. speed on a century is around 20 nowadays - but I can get down into the 13MPH range on a good steep hill, and up to 45-50 on a good downhill grade.
Vary your seating a bit; I personally do all my training still on my cafe-style racer with an aerobar, and I may actually show up to the HHH on that particular bike. I've found it offers a lot being a 90% race bike (upgraded to 105 all around except the controls which are still Deore) with a bontrager straightbar, bar ends that are flat and an aerobar (Most say the bike reminds them of a TT bike). I've flipped the neck to give it a negative angle and it offers me a nice downward angle very much like my Specialized Transition, without being stuck with a strict ramhorn style bar w/ aerobar bike all the time. - Actually, ya - I will be riding the Del Sol I think in the HHH.
Anyhow, the biggests trick I've found is that any time I intend to ride for over 3 hours I start at 6 (right before sunrise) and have consistently training in the 6 a.m.-noon window when riding. Conversely, all my running mileage is done at night (11:30 a.m.-4a.m.) simply because I've pushed it a few times running high mileage in the intense heat.