You'll absolutely make it. It sounds like you bonked on your last ride. Eat a good breakfast that you know agrees with you and bring some form of energy -gu, clif blocks, gummy bears, whatever.

I rode El Tour de Tucson in November - 109 miles. A few weeks before I had an absolutely awful ride - I could barely make it 60 miles. I was ready to give up then. But the next few weeks I improved and I felt better about it. Then the week before I had another awful ride, just like you. But the actual event went great!

There's no doubt in my mind you can finish the century. Don't put too much pressure on yourself about speed or finishing time and you'll be fine. You have PLENTY of training under your belt.