Awesome on losing the 80 lbs. I thought I was doing good losing 25 over a 10 month period. Being from Central Florida, we have the heat and humidity almost all year long. Before I started cycling, I could stay outdoors for quite a long time and handle it pretty well. However, I wasn't doing any type of exercise or much of anything else except walking or just standing. If I did anything other than walking or standing, I would really feel it and had to go inside and cool down within a matter of minutes. When I started cycling, almost two years ago, I had the same issues that you are having now. I'll be 65 in October. I don't know if this will work for you, but this is what I did to help overcome the heat and humidity:

I started riding short (5 to 10 mile) rides everyday in the late afternoon when the heat was at it's highest index. The rides started off slow paced (6-9 mph) in the beginning and increased as I got more comfortable with riding in the heat. The longer, harder rides were done in the early morning when it was much cooler. I can now ride 2 to 3 hours in the heat and humidity and not get any cramps or other problems other than it can wear me out at times if I get too aggressive in the ride.

I purchased a clamp on water bottle cage for my seat tube that I use when I ride distance rides in the heat. This gives me three bottles that I have to draw from. One bottle is plain water, the second is electrolytes (I use ZippLite that I get from Sam's Club) and the third is a mix of water and Hammer Perpetuem. I thought about a Camelback but then decided against it as I didn't want that extra weight on my back while riding a road bike in an already angled position. I constantly fill the water bottle so I always have water and I also take an extra tube or two of ZippLite on the rides so I always have electrolytes. The advantage of an electrolyte mix over Gatorade is that when the Gatorade is gone, you either stop at a store and get more or you are out for the rest of the ride. The mixes come in small tubes and you can take several of them with you. The Perpeteum is a quick recovery drink that I take a few sips of every 30 min. or so. It also reduces the lactic acid that causes the leg cramps. I also do a short leg stretching exercise at each stop that lasts more than 5 minutes. I use GU Gel packs and GU Chomps during the ride for quick energy boosts. All this goes into a small handlebar bag that other roadies think looks dumb on a road bike, but who cares what they think.

Also, I checked with my doctor and did a blood test to see where my potassium levels were at. Since they were normal, I started taking 99 mg of potassium each day. I don't get the leg cramps any more, on or off the bike. Make sure that you check with your doctor before taking potassium supplements because it could cause hyerkalemia, which causes cardiac arrhythmia's.

As I said, this is what worked for me but you will also hear what worked for others, as well. My opinion is to try one or more of the suggestions and see what works for you. Try combining some of the suggestions to see if that works even better for you. Don't rush it, you still have plenty of time to get in the training for the century ride. Get yourself acclimated to riding in the heat and high humidity and the rest will follow. Don't concern yourself too much about falling behind younger more experienced riders every once in a while. At some point in their riding history, they were where you are at now. The objective of most of these group century rides is to finish and crossing that line is one hell of a feeling of accomplishment.