5 hours is pretty fast for a solo century.

Given that you struggled a bit to finish 80, in lesser heat, I think you'd be well served to dial back you pace, and make your goal finishing the 100 miles without worrying about the time.

Start out a bit below you average speed you did for the 80 mile ride. If you're feeling good at 65 miles or so you can always pick it up a bit.

Conversely if you go out too hard, and only start feeling that at 75 miles or so, the last 25 miles can really suck.

And as for eating and drinking, do as much of both of those as you can get down, starting early. Standard wisdom is most people can consume about 24-30 ounces and hour and 250-300 calories.

You're going to be running a deficit in both regards, so the trick is to keep from falling behind as much as you can.