I do centuries all the time in RI and and southern mass. This is one of the flattest places around. Lots of good cycling. The highest point in Rhode Island Is 900 plus feet. The best is probably Martha's Vineyard Mass., an island. (google it for a bike path map and information, you can rent a bike on Martha's Vineyard.) There is a RI state bike map you can request on line. (Bike R.I.) I have a really nice Rubels map of Massachusetts and all below Boston and out to the cape where I ride. People think my local photos are from N.H. and Vermont. It's less hilly here than N.H. and Vermont. They have lots of good cycling too. Using Boston as a base is a great way to do some riding. Cape Cod has lots of nice cycling.
Send me a P.M.
I can link you to some of my ride photos.
Google "R.I. East Bay Bike Path" to see my local area. There are links to it on "Bike R.I. too.