78 miles really isn't that long.
"Cardio" is good, but won't help you all that much on a bike. Your ass is going to hurt. Probably your shoulders too if you don't ride outdoors, because (that means probably) you don't have your bike fit dialed in, and that adds up over longer amounts of time in the saddle. As mentioned, it will take you longer than people who don't look as fit.
Intervals are not the way forward with this, you want to build endurance, taking a rest every few minutes isn't the best way to do that.
Indoor bikes aren't a good way to train for this, for a lot of reasons. Am important one is indoor bikes don't teach you anything about handling a bike outdoors.
If there will be a lot of other cyclists around, you should do some group rides between now and then to learn a bit about riding in a pack.