There are a growing number of sponsored rides in Maine (or at least I am noticing them now!). The century you refer to at Acadia might be the Tour de Cure (might be a metric century). Also another ride big in numbers and miles is the Trek across Maine in June every year. It is a 3-day, 150 mile event from Farmington to Belfast, with 2200 riders, always subscribed well in advance. Website for that one is www.alam.org
The Bike Coalition of Maine has a very user-friendly calendar which is put together every winter. www.bcm.org
Hope that helps. Come do a ride with the senior roadies here, Veronica!! May see you there!