I have not been on the AT for more than 6 days in a row. For the terrain I have experienced (all south of Dickey Gap) I average between 1.5 and 2 mph (including breaks). 2 mph ave for a day is pushing it unless it is a smooth section like Hogpen to Unicoi. I just don't have the aerobics to fly up a hill and have to pick my way down rocky declines to avoid injury.
10-15 miles is a comfortable range for this section hiker.
If I was starting a thru I would do LESS miles in the beginning and expect to be in the high teens in a month.