If I just wanted to be able to say "I did it" even if I did it badly .... I'd just go for it.
Maybe I would send the last 25 miles hating myself, life, cycling, everyone who asked it I was okay, and all the parts of my body which really hurt sharply .... and maybe I would have to stop every five or seven miles in the last 25, because it hurt too much .... but I'd make it. Or not.
If I wanted to Enjoy a century, I would take my time preparing and make sure i was ready. it might be a seven-hour adventure, so I would need to plan for changes in weather, and Lots of food and even more water and/or sports drinks. I would probably want to consider restroom breaks and where they could be done conveniently. And it depends f you wanted to be as much as possible non-stop, or a 20-minute break at 70 miles was okay with you.
I am too old to ride to prove stuff. I know I can hurt myself and I tend to regret it more and recover Much more slowly as I age. if you are fit and young, you might lose a few days to stiffness and soreness and fatigue. I might lose a week or more before my body was ready to ride again---which I why I would rather prepare more and push myself less on the day of.
I like to enjoy my rides, even if I am enjoying riding hard .... which I cannot do for long .... so for me, I'd do the 68 and work up to 100 over a longer time. But maybe you have the capacity to do it and not regret it. Who Knows.
I find my lower back is where I get the worst and sharpest pain. Neck and shoulders feel stiff and sore but the lower back muscles really scream at the end of a near-capacity ride. My legs get tired and sometimes cramp (which really sucks) but the only unbearable pain is usually my lower back.
Keep us updated please.