I need at least one 50-miler before even considering doing the actual century. 70 miles during training for a century is best for me. You did only 30, so 75 miles is actually quite good. Congratulations.

(Actually, I find titme-in-the-saddle to be the most important factor; not so much miles.)

It's not the legs that get you when you exceed your training distance by 2x or more. It's instead the things you said: shoulders, and neck, and butt ... all of which get hardened with training and allow you to push through a century.

Good luck on the next one.