I found your thread in Road Cycling. Waytago on the 50 miles! 16.1mph is a very good speed too. The longer you go the more you'll stop occasionally and the more cautious you should be about having enough energy to finish.

The bit about having chills when done doesn't sound good though. You do need to drink water, especially on real hot days. Using more GUs or whatever they are called probably isn't a good idea. Imagine drinking 4 cups of coffee...the first will work, the second and third might, and the fourth will make you sick. Since caffeine is a diuretic it will further dehydrate you. Better you should eat a banana, or mix something with more water. Find a convenience store or fountain if you don't have another bottle. Eat a good breakfast before you go.

I don't have a bottle cage on my bike (though I'm considering it). I carry a .75liter bottle in a handlebar bag and it is usually in an insulated sleeve. Inside the bar bag it stays cool for an hour or so if I start with ice water. Another good trick is to fill your bottle half full the night before then freeze it, trying to set it in the freezer on an angle so that the ice doesn't freeze the top shut but extends up one side of the bottle. This increases the ice surface area. Just before you leave for the ride fill it the rest of the way with water. You'll have ice water for a long time that way, especially if it is insulated. With half of it being ice you'll drink all the water part pretty quick. Refill as required, which will melt more of the ice.

But you see, I can get away with so little water because there is plenty on my usual ride. There are two fountains on the Minuteman path in Lexington, one at the end in Bedford, and one in Concord Center. If it is a Saturday I can go into the Carlisle library in Carlisle Center. Then there is a fountain at the North Bridge historical site. Water, water everywhere! Not counting the convenience stores. Drink! And eat snacks. Power bars, dried fruit, stuff like that.

Ride on!