It's not an easy route for a first timer, but I can't say it's not possible.
For these kinds of events, if you're fit and you've been doing a fair amount of training, you're racing against yourself and trying to do your best. Overall placement and by age group in these kinds of events are just bragging rights and mean absolutely nothing beyond that.
Whereas if you signed up and haven't ridden more than 10-15km a week, then you're racing against the clock, to finish within the given time. Take note, you are not rewarded a finishing medal if you don't finish within the time limit!
You got plenty of time to get ready, so don't worry. You also did that imperial century not too long ago, so if you can finish that, you should be ok!
For race day...
Arrive early (30-40 minutes before start). Two bottles, one of them with supplements or sports drink, it'll be hot hot hot and water stations will be spread out. Bring some supplements with you in forms of gels or bars. Anything to keep you going and the stomach from not going empty.
The fast guys will all be bunched up in the front at the start line, so if you have confidence to keep up with them, no one is stopping you and no one will laugh if you get passed. Since...you'll most likely get passed.
I wouldn't suggest staying in the back back of the pack since you'll just get stuck with people thinking they can do long distance with lots of elevation gain in the heat from doing leisure riverside rides every evening. Trust me, there's always people like that at these kinds of events.
What's fun about these kinds of events is riding with a bunch of people. You may rely on people of similar pace as you to get you to the finish! So you definitely don't want to start too far back. You'll end up spending too much energy trying to catch up to those with similar ability as you.