I need to clarify why i said this. people have said that it is not a big physical effort doing 100 especially if you pace yourself so you should be capable right now. I suggested doing a 60 as a test rather than necessary training because I too think it is not physically difficult if you pace yourself. The reason you need to do a 60 mile test is to test your bike fit. Often in 30 mile rides you will not notice a potentially dangerous bad set-up, but i find that 60 miles it will be shown up.

For example, I got a nasty lingering hamstring injury in 2009 because i went out and did a century as soon as the winter bad weather cleared. I was on a newly built bike and even though it felt great, the seat height was wrong by a few mm and over a 7 hour ride, I strained my hamstring tendons. I had only done short rides testing out my newly put together bike.