I'd say you were there unless the 60 mile ride has significantly more elevation gain than what you are currently riding. I just rode my first metric century at the end of July. In June I rode 9 rides for a total of 136 miles. July I rode another 9 rides for a total of 257 miles. Prior to the 62 mile ride I had never been on a ride over 34 miles. The route I was on had less elevation gain than the 33 mile route I was riding, so that helped me a bit. I was very pleased with being able to finish and my 15.8 mph average speed. Not to shabby for an overweight 44 yr old in my book.

I would say ride at your pace and have fun. Make sure to hydrate and fuel along the way. I used Hammer Nutrition's Endurolytes Fizz, Perpetuem, Gel packs and Recoverite. They have a lot of good information on their web site. There are numerous alternatives to those, but that is what seems to work best for me.

Good Luck!