New England mountains are a different creature from West Coast mountains, but the Westfield 400k goes over Brodie Mountain Road and past the Jiminy Peak Ski Resort on both the outbound and the return legs, and the Boston 600k goes through the Green Mountains in Vermont on both outbound and return. Our rides out here tend to focus more on series of steep, heavy rollers; though either mode have their proponents and detractors. I rode part of the Boston 600k with an RM1200 veteran, who infinitely preferred a steady, long mountain gradient to all of the up-and-down stuff that we have.
Enjoy the 400k. Personally, it's my favorite distance. An entire day out on a bike, long enough for the entire affair to feel like an adventure; but doesn't eat up all of one's weekend.