I love Huntington, myself. Traffic flows pretty smoothly and yet it tends to be light enough that folks can get around you if they need without too much hassle. I take it twice a day most days. South Huntington kind of sucks because of the tracks and the descending/climbing of which I'm not a fan.
Mass Ave in North Cambridge all the way into Arlington and beyond is pretty flat and a nice spin. You can fill out mileage pretty easily that way.