In 2002 I did my first AT section hike and averaged 17 miles a day between Springer and Damascus, but most of the days for the last 300 miles were 20+. This wasn’t especially difficult. On the PCT, I averaged 25 miles per day over the whole trip, but much less in the beginning. Over the first 1150 miles, I averaged 22 miles per day. In 2004, I went back for an 1150 mile section on the AT and averaged about 24 miles per day. This was Damascus to Manchester, VT.
People end up averaging fewer miles on the AT because they take a lot of time off, not because it is any more or less difficult than the PCT. While the AT has steeper graded climbs, they are shorter than those on the PCT. For example, how many 3000+ foot climbs are there on the AT? About 4 south of NH. 3000+ foot climbs are common on the PCT. Even though the grade is moderate, going up hill for a long time can be tiring. The AT does have very brutal sub 1000 foot climbs and these add up.
Again, though, there are towns everywhere, hostels everywhere, shelters everywhere on the AT, which encourages people to take a lot of time off. Some will call it “smelling the roses” but it is really just down time. 20+ miles a day is pretty easy on the AT, just be aware of the short, savage climbs. You may have to really work for 20 or 30 minutes to get up to a viewless lump and then come down the other side, only to repeat it six or seven times that day. This does add up.
Suge