I think you've gotten good advice so far. I'm surprised, however, that you are attempting to tackle 75 so quickly after your first 54. If you have trouble with it, it could be because you haven't recovered from your recent long ride, so keep that in mind when assessing. Generally doing one long ride a week gives your body time to recover, with shorter rides during the week and a "recovery" ride the day after the long ride or anything particularly taxing on the body. (Recovery ride is just a super easy ride where you spin your legs for 30, maybe 60 minutes. In a book I read it said to ride slow enough so that a little girl with a basket on her bike can keep up!)

If you are good after your 75 today, I'd do a recovery ride tomorrow if you can, or even a good slow walk will help. Then wait until the weekend and do another 75 or maybe 85, and then a recovery ride and maybe another moderate to easy paced ride or two during the week before your century.

Make sure you educate yourself on nutrition and hydration, as that can play a big factor in your success and misery level. Don't forget about post ride nutrition for recovery, not just for the century, but for your training rides.

Just being on the bike in that position for 6 plus hours can be difficult when you are not used to it, even if you have the strength and energy, so be prepared for that factor and hopefully you've had a professional bike fit.

Lastly, when doing a century, I find it helps to think of it in shorter spurts. Think about the 20 miles you have to go to the next SAG, not the 60 you have to go to the finish.

The last 20-ish miles can be a big challenge, but once you've finished 80, while you may be suffering some, you'll know you can get in that last 20 and you'll have finished your first century. You'll be so proud of yourself. I have feeling you'll be just fine.