this?
Blue Water Ramble - Clinton River Riders
Looks like a great time. For any organized ride like this, fully expect helmets are required, as well as waivers and commitments to abiding by rules of the road. Also, most of the time, you do the route you signed up for. Some rides may loop the distances and you could have a mid-course decision, but they also might charge differently for different lengths. For example, one of the staple rides around here has a fork in the course for the 62 vs 100mi routes. You can choose when you get there depending on how you feel. Not all rides are set up that way though. Looking at these routes, all of them longer than the 27 do the island first, and then more. You might be able to add on if you're feeling it.
If you have a chance to lengthen your rides over the course of a couple months, there's no reason why you couldn't do a longer distance. (stick to the shorter rides during the week, and go for a longer distance on the weekend) Would be a great goal
Good luck!