I did a flat century, but with vicious headwinds, with no training other than my usual hilly riding, with rides up to 65 miles or so.
Would I do it again? Maybe. I might want to do a 75 miler or 80 miler, before, but I don't think that would take away the mental pain of miles 85-100. Most of it is in your head and good nutrition/hydration. I'm just really adverse to "training." I knew in my head I could do it, and I set a realistic time goal, which I beat by a half hour. I did not ride fast, with 6.5 hours of riding time.
I had been riding for awhile when I did this, so SkyWalker is in a different position. But, it seems like maybe you need to do *more* riding, not necessarily longer. There's a huge difference between 30 and 100 miles. The year I did the century, I rode 3,000 miles, but a lot of those miles were little 15 mile rides I could squeeze in before or after work. I always ride both days on the weekends, usually one long ride and one of 15-25 miles. I take Mondays off and another day during the week. Mostly, those are active rest days, doing yoga or walking/hiking.
Everyone's different, but I would start riding more if I was in your position, while at the same time, slowly increasing your long ride of the week.