Can't answer that question for you but I'd expect to get farther than that for myself. What sections are you familiar with?
Here's a quick summary of that stretch plus more:
* the first mile or so from Caribou wayside is up hill but then it's relatively easy hiking until Cross river (about 13 miles)
* fairly hilly from Cross River to Briton Peak (about 8 miles)
* very easy hiking from Briton Peak to Oberg (about 5 miles)
* quite hilly from Oberg to a mile past the Caribou Trail (about 12 miles)
* relatively easy hiking the rest of the way to Cascade
* the first 4.5 miles after Cascade River are quite hilly on the east side of the river, I have not hiked the west side
* it's been a decade since I hiked from there to Grand Marais but I was a less fit hiker then and don't recall any great difficulties.
One issue for the average daily distance you are proposing (7 miles) is the 9 mile stretch between the South Cross River campsite and the next campsite at Springdale Creek (unless you reserve a site at the Temperance River car camping campground)
For comparison, I'll be with a not SO strong hiking group going from Finland to Temperance river (about 34 miles) in 4 days next weekend. That includes driving there on day 1 and driving back on day 4 (from the twin cities). Half of our route overlaps your's.
Sounds like you are finishing up on a Friday, if you are planning to leave your car at Cascade River you could use the Superior Shuttle to get you back to the car if you end up going past Cascade (shuttle runs on Fri, Sat and Sun)