Long and smooth? You can essentially ride from the bridge to PTown on Cape Cod. There are miles and miles (like 30 something) of smooth flowy trails at West Barnstable Conservation Area (AKA Trail of Tears) between ol' Exit 4 (Chase Rd.) and Exit 5 (Rt 149). Lots of little punchy climbs and no real super tech stuff other than a few rock gardens here or there. And to the doubters, you can easily get 1200 ft. of climbing in a couple hours there. Across the street on Chase Rd. there is the Town of Sandwich's version, it's hillier, a bit more challenging and fairly smooth as well. I sometimes switch it up and enter through the Maple Swamp trailhead on the Service Road. If you are nearby the Cape, it'd be a shame to not check these spots out. Make a long weekend of it and throw Otis in there too. Park at the intersection of Rt. 151 and 28. Head north or south of 151, you'll probably see riders heading in both ways, I prefer the north side myself. And if you do make it, and want just a little something else, Nickerson State Park in Brewster has a really fun 14/15 mile loop with pine needle cush flowy trails aplenty around the lake. Believe it or not, all these riding areas connect with minimal street riding!!! And if you have a gravel bike treat yourself to the Cape Cod National Seashore trails. 50 miles of spectacular views and you'll mind will be blown that there is that much wild area way out on the Outer Cape.