Ive been road cycling for over a year now. I am capable of riding 57 miles at 18mph with 2k feet of elevation. My goal is to be able to do a 100 mile course in about 5 and a half hours next year. Ive done all sorts of training from long and slow rides to short and quick. But what is best for the given goal? most of all, is it a reasonable goal?

Totally reasonable. Try riding that mileage with twice the elevation gain if you can find it. Gradually increase the long ride mileage. Learn to "titrate the pain." You want to go as hard as you can for the distance without quite bonking. On the shorter rides, say up to 60-70 miles, try taking all the longer climbs at lactate threshold. There are two specific helpful intervals you can do: hill repeats at various hill lengths, say between 4 and 20 minutes, and speed work, meaning to ride at lactate threshold at high cadence, say a set of 4 intervals of 15 minutes, something like that. That said, I've never been an interval guy, preferring to get my intensity during my long rides. Just my preference. My experience is that to ride a fast century, you want to have about 2 months of 200 mile weeks in your legs.

Make sure to include one long ride per week in your schedule and then make sure you taper properly before your event. Sometimes your time off the bike is worth more than time on before a big ride.