I estimated by my log from my section hiking that you would have elevation gain of 18650 ft from Springer Mtn to Bly Gap. I took 7 days to hike it, so an average of 2664ft per day if no zero days. My estimates are only from the reading of the profile map for the GA section when I hiked it in the spring and summer of 2000. I did not include the approach trail. The elevation of Springer and Bly gap look to be about the same at 3800ft +/-, so the elevation lost would be about the same as the gain. I thought the grades back then were pretty steep compared to what came later, but that may just be the result of better physical conditioning or a better mental understanding of what would be involved. Also, switchbacking becomes more prominent later. I have not hiked north of Harpers Ferry though, and understand the Whites are considerably more difficult from what many others say. If you can do 2mi per hr on the flat, and figure an extra hour for 1000 ft of elevation gain with a total daily mileage of 10-12 mi, you are plenty ready.
Cutman11