Trevor, the Allen Gap to Roan section is lovely. Bit of a grunt from Hughes Gap to the top of Roan, a couple of less severe but still "know it's there" climbs from the Nolichucky up to Curley Maple and up Unaka Mtn, plus a long, steady up to start with, but they're all quite doable.
The whole package might be a bit long unless you're quite fit and determined, though -- isn't Allen to Carver's around 85 mi? While there are some lovely views between Allen and Devil Fork gaps, starting at Devil Fork trims about 20 miles.
I might be more tempted to start at Sam's Gap and do the 71+/- mile trip to 19E (do NOT leave your car there!), picking up a nice trip over the famed balds of Roan and still scoring more than 18000' of ascent. That section offers a bit of everything -- balds, climbs, rhody tunnels, open forest, ridge walks, views of the Nolichucky, wide panoramas... I like it. You would miss the Shelton gravesite and the cliffs around Whiterocks, though.
Not sure how you're working shuttle, but I think Uncle Johnny's in Erwin will take you to Sam's for around $20, and could pick you up at 19E as well.