I've done 580 miles on my KLR on 6 occasions. Albeit I had to be at a destination on a specified day. The AT being far more comfy and freeway capable the same route would be easily doable. However, if time wasn't an issue, I would have rather done 300 in a day then have time for sight seeing and general lollygagging.
If you don't know where you're going, don't go there fast.