700 feet elevation gain per miles is the toughest non-AT climbs I've done - both Tearbritches in Georgia's Cohutta Wilderness and the North Rim of the Grand Canyon, starting at Redwall Bridge, climb about 2,100 feet over three miles.
700 feet per mile is darned tough.
I've done the AT from Springer to The Scales in Grayson Highlands, Virginia. There is no three-mile stretch like those two. I don't think there's a two-mile stretch. The steepest climb I've done on the AT is Jacob's Ladder, which may approach 700 feet over a mile. The second steepest is probably Albert Mountain, but that's just one-third of a mile. Most of the other long, tough climbs are 500- to 600-feet per mile, I think included Wesser to Cheoah Bald. There's a big difference between 500-feet- and 700-feet per mile.
The northbound climb up Roan Mountain used to have a fearsome reputation, but it had been modified by switchbacks when I arrived in 2015. It wasn't that bad (I'm guessing about 500-feet per mile). Before the modifications it must've been a booger bear.
Bottom line is this: for me, at age 57, I don't worry about 300- or 400-feet per mile; I know 500- to 600-feet per mile over an extended distance will hurt; at 700-feet per mile, I "gird up my loins" for a bear of a climb.