I searched on 'training plan for century.' This was the first result (I haven't vetted it, but you get the idea):
https://www.bicycling.com/training/a...ide-a-century/.
If your century is part of an organized ride, I think you'll have 12 hours to complete the 100 miles, so averaging 10 MPH will work. Still, you might do yourself a favor if you sped up.
Best of luck - I hope it's a sunny, warm but not hot, tail wind all the way....