Pamestique 

You can do it... just keep riding and trying to push up the mileage to at least 20 miles. Remember you have all day. There will be sag stops. Remember to stop, eat (an orange slice, a fig bar some pretzels - not much - just enough to keep fueled) and move on - don't dilly dally much. Just keep pedaling. You will be surprised how the excitement of the event cheers you on.

I did my first century ride after having only ridden 55 miles once before. I went with some friends and planned to stop at 50 and sag in. Well (1) found out too late, the ride had no sags (oops!) and (2) the event was so exciting I just kept riding. Before I knew it - I finished 108 miles. I was only 33 minutes behind my friends who had actually trained for the ride!!!!

Remember long distance riding is 99% mental... keep it in your head this is going to be fun and you will be done before you know it!!!!

You mentioned you don't like hills... I suggest you pick one out you don't like or has been a challenge for you and go ride it. Take your time, low gears are your friends. Just complete the climb. Realize climbing is no more difficult than flats it just takes different gearing and attitude. I used to hate hills so spent one whole summer doing nothing but. Now I don't like rides that are flat!

Also one other suggestion... check out the route, just in case you need to cut mileage, at least know where you can.