It depends on your goals and what you mean by spinning. I cycle and take some spin classes. I see people in the classes who look as if they are working hard but are not. Some of them even do a a pot load of classes. Some people are not doing that much in spin classes and others are working hard. It is hard to say what you are doing. You call the classes "grueling" but that depends on your vantage point. Spinning at the proper intensity would be a great way to prepare for a metric.
Now you have ridden 42 miles and that is an excellent training distance for a metric. The great thing about doing a ride of a certain distance is there is no way to fudge. The main thing is do you think you can go 50% farther.
Spin classes tend to be short and intense and they keep changing the tempo all the time. Road riding is much more a matter of getting to a certain cruising speed and holding it.
On the big day, it will be easy to try to push yourself too hard. Take it easy. Ride within yourself. If you get to 50 miles or so and have energy to burn well then let it rip. I would also suggest that you eat some light snacks on the ride (bananas, oranges, and other fruit are all good) and drink frequently. Since this is a first time, I bet they have SAG stops. I would suggest breaking the ride into 3 segments. Stop at mile 20 and 40. Stop briefly. Fill up your water, eat a snack, relieve yourself as necessary and walk around and air out you contact points especially the hinder parts. Do not stop very long otherwise (about 5 - 10 minutes) it will be hard to get going again. Also do not change any basic gear (shoes, shorts, bike fit) the day of the event. Go with what you and your body knows. Good luck and let us know how you did.