I did my first metric century in late March. I found out about it and started training for it in mid-December.
In mid-December, I was riding my cross bike, typical rides were 15-20 short with one longer 25-30/week.
In Jan, I increased those with the short 20-25 and long 30-35/week.
In Feb, that went to 20-30 for short (depended) and long 35-45.
In March, short went 25-35 miles, with long rides 45-60 miles.
I rented a bike (a Specialized Roubaix) for the metric century, averaged right around 18, and didn't have any major problems. I just stopped once, at the 3rd SAG, somewhere around 48 miles. I remember they had animal crackers out and I devoured them.
I had been a distance runner, and mostly followed a similar plan as I had for ramping up before a marathon. And I plan to follow something similar before a 111 mile tour this fall, with some rides in the 90s, and at least 1 around 100 miles.
More important than eating during, I've found properly eating and drinking that morning really helps. Before each of my marathons, the metric century, a 70 mile tour, and all long runs/rides I eat a PB&J sandwich. It seems to have the right mix of carbs and protein to keep me going. It is extremely rare that I need to eat during a ride now...even some 75-78 mile solo rides.