There is no 100 miles between Buzzards Bay and P-town and back. And while none of the hills are large, it rolls and rolls and rolls and rolls.
Seriously take a look at the Bike MS and PMC routes. Well published, easy to follow. Since you won't be riding with a few thousand other riders, you can err on the side of riding the rail trail farther (pretty much between East Harwich to Welfleet.) Past Truro you will be riding either 6 or 6A, but otherwise there are far more pleasant roads to ride even if they add a couple of miles. (There's one stretch east of the Welfleet fire station I swear must be paved with sea washed stones.)
Finally, don't underestimate the wind. It's the cape, it always seems to shift so that it's always in your face. (The cycling gods are being kind to you if you are riding a cross wind, and you've been completely blessed if you catch a tail wind.)
Last edited by mr_bill; 08-07-14 at 10:49 PM.