DH and I just did a metric century (100 km; 62 miles) in 3 hr. 50 minutes. It was wonderful - a lot of fun - but hot. Start early if it's a Show-and-Go and if you are in a hot area! That was my 2nd "official" organized metric ride -- I've done that distance several times before, or more, but not as part of an advertised metric.
My first ever full century (160 km; 100 miles) was about 7 years ago, and it was incredibly flat and I did it in 5 hrs. 20 minutes (yes, I still remember the time!). Weather, injuries, and general conditioning have never let me beat that time since then. Beginner's luck, I guess.
The full centuries (other than my first) I specifically rode with that ride in mind - so I opted for long rides on weekends and when I could. The metrics were both kind of spur of the moment things.
My tips:
Ride lots before the ride. I would guesstimate that you should be able to do at least 2/3 the advertised length before the ride to finish it with any confidence. There's a lot to be said for the distractions of an organized ride - the people-watching, the rest stops....all help to make the miles go by more quickly.
Speaking of...Rest stops are always good, but don't spend too much time at them, or you will tighten up and lose the motivation to keep going.
Pace yourself. It's not a race and if you burn yourself out in the first 20 miles, the experience will be dreadful.
And the biggest of all: have fun.