Like the OP, I had this idea that someday I'd like to do a century. I thought of it as a "bucket list" item. Then somebody posted on these forums that rule of thumb that you should be able to ride in a day as far as you usually ride in a week. I was riding a bit more than 100 miles a week, mostly in 15-20 mile daily rides, and that rule of thumb made me think I could actually accomplish that goal. I'm not passing judgment on that rule of thumb, but I rode a flat easy course and stopped every 15 miles or so, and I did it! That was last year, when I was 70 years old. I plan to do another easy century within the next month, when the weather cools down a bit.