I did my first long ride as a charity ride with a group after riding about five/six months. It was 48 miles. I also used a hybrid bike but put slick tires on it. I worked up to it, my longest ride was about three weeks before the charity ride, at 37 miles. It was a tough ride for me and I ended up riding alone most of the time because I am slow. Having the rest stops with food and water helped tremendously. The ride was flat and there was almost no wind, which also helped a lot. I also was helped by the fact that there was a 25 mile group that started after my group so there were plenty of people around and it wasn't like I was the last person to the rest area, coming in late and discovering that all the food was gone. My ride had a recommendation that you be able to average at least 12mph, otherwise you might not finish before the rest areas closed. I wasn't quite averaging that speed before I did the ride. But riding a group ride does make you faster so I averaged above 12mph, actually close to 13mph.
My rear end got a bit sore and I certainly was glad to have bike shorts.
I am much older than you and totally lacked any fitness when I started biking . So, for me I seriously doubt that I could have ridden 40 plus miles without having ramped up slowly over several months. I think a younger person who is more fit might be able to bully there way through such a ride. Or not. You might end up being just too uncomfortable on the bike with that length of time in the saddle. I agree with those who suggest trying out a 30 mile asap and see how it goes. It is simply too hard for us to guess whether the ride makes sense for you. I remember that you had problems with the fit of your bike only a few weeks ago and your bike isn't quite the right size. That might also be a limiting factor for you that could show up on a longer ride.
Training for the metric is a great goal! I did my first metric this June and was in good shape for it. It made the ride a lot more fun than my ride last fall.
Trek Madone 4.7 WSD
Cannondale Quick4
1969 Schwinn Collegiate, original owner
Terry Classic
Richard Feynman: “The first principle is that you must not fool yourself and you are the easiest person to fool.”