Congratulations on the ride--that's a big achievement for a newb.
Your fatigue is pretty normal, I think. I did my first century at age 44 when a friend called on a Friday night and asked me to ride with him the next morning. I was a casual cyclist, 20 to maybe 40 miles a week, with a lifetime long ride of 30 or so. I'd recently run a marathon, though, so I was in decent shape and agreed to go.
the first 75 miles went great. We just cruised along, not too fast, enjoying the scenery. By 80 I was completely hammered, barely able to move the pedals. I had a flat about there, something I can normally deal with in four or five minutes, and it took me almost half an hour to fix it. We struggled on somehow, finishing the hilly course in about eight hours, and I felt like absolute crap for two weeks. I had no energy, fell asleep at work, couldn't ride...
Ride more, building up slowly, take rest days now and then, stay hydrated all the time. You'll still feel like crap at the end of a century, but not as MUCH like it.