You have come a long way in a short time period! It has taken me since spring to work up to 50 miles, which I did in my first charity ride last weekend.
How slow are you? I am very slow and an older rider that was obese and totally sedentary at the beginning of last winter before I started riding. I averaged 12.2mph on my 50 mile ride. My speed was no problem and I was not the slowest, though I was one of the slowest. They had different start times for different groups (110 miles on down to 22) but they were not at all rigid about when you started, you just needed to be done before all the rest stops closed. Given that you already have done rides longer than the 27 miles odds are you will be fine. You may also find that you are faster riding with others. Previously, my longest ride was 37 miles and I averaged of 10.5mph on that ride. It was on the same trail a month before. Riding with others helped speed me up a bit.