It's never too late to start. I don't know how it is in SI, but I frequently see folks in their 60's on some of the longer rides. I can't speak to the Misaligned Minds ride, but my general knowledge of the area is that this is a good place to do your first century. Not too difficult and I believe there is typically little or no wind at that time of year, though heat may be an issue. It sounds like you're already well prepared.
There is very little difference between riding 75 and 100 miles. Especially since this is your first century, you might actually find it easier because first centuries are exciting. Let us know how it works out and good luck!