well if you are currently doing 13 miles in an hour (leisurely) then 60 miles is going to take you say 4.75 hours if you maintain the same average speed. Realistically you are going to get a bit slower so I would suggest depending upon hillyness about 5.5 hours including a couple of stops.
Thats a lot of hours to be supporting your trunk .
You'll probably find that you'll get saddle sore and your neck and back will ache. Your hands will go numb and it will start to feel like slow torture .
That's the good bit
If its raining ,hilly and you get some punctures .?....well you get the picture.
Have a look at the events page on the Audax site.
http://www.aukweb.net/cal/index.htm
I'm doing the 'Dave's Doddle 100k in April at Bishops Lydiard, my first Audax for 12 years
You need to get in some 40-60 mile rides in to build endurance and strength and see how you cope . You may find you need to make changes to your clothing, gearing and bike setup.
If you are managing 30 miles now you should be fine if a little tired and sore the following day. make sure you carry plenty of glucose rich food and snackbars and put some electrolyte in your drink