I haven't been on the AT, but even out here in Oregon I have heard of its reputation for PUDs (pointless ups and downs), so it seems reasonable to assume this will be the case in your section until proved otherwise.

Given your fitness info, you should be safe to figure a ballpark of 8-10 miles a day 'safe' in the sense that, even if it turns out harder than anticipated, you still could make your miles each day by pressing on. If it is easier than anticipated, you'll have that much more energy to putz around, explore side trails and play games during the day.

Trail conditions do make a big difference. To give you an example, I can do 10-12 miles a day on the Pacific Crest trail (PCT), but I recently came back from the Eagle Cap wilderness in eastern Oregon, where the trails are rutted up, rocky, and climb over mountains on a regular basis to get from one river drainage to another. There, 8-9 miles a day was much more realistic for me.

Trails in the Glacier Peak wilderness in Washington State are even more challenging and I would reduce my miles accordingly.

You'll just have to go with a ballpark figure and adjust as you know more. Low-balling your estimate might be the way to go, unless you get all bored and frustrated if you aren't sweating like thunder and blazes.

Happy hiking!