thats a nice loop, and yes 320km and 1000m climbing is nothing, very doable.
Ive ridden parts of this route on diff rides over the years, nice area to ride in and the Champlain islands road is a nice ride, as is the section to Montreal, although I havent specifically ridden those roads or maybe la route verte you did up from Burlington to here in Mtl.
When I did Mtl to Boston, I made my way down to Lake Champlain on quiet back country roads mostly, down to Rouses Point near the border in NY. Then from Rouses to the island, to Burlington and then onwards down Vt.
very pretty riding, although it does get hilly after Burlington naturally.