Listen to Indy about the warm up. Even when I started riding (and I was about ten years younger) I noticed that I never feel good until I have been riding for about 7-10 miles. Which means on my short rides, I have to really go slowly in the beginning. This sometimes is a problem for me on group rides, where they often start out like a bat out of hell...
And what Marni said is true. If you can ride 40 miles, you can probably ride 50 or 60, albeit, at a slower speed. I did a century with no training, other than my regular riding. I had not done any rides longer than 60-70 miles. I knew miles 80-100 would be tough, but I set a very moderate speed goal and ended up finishing in 6.5 hours.