At my current age (late 50's) 10 miles with any significant elevation change is a big day - 8 is more common, with lots of stops to smell the flowers.

When I was your age, hiking in the Cumberland Gap and southern Virginia area (count on 2 or 3 elevation gains and losses of 500 - 1500 feet each per day), I usually averaged about a mile and a half an hour with a 45 pound pack. The best I ever did, with a 30 pound pack, was 2 miles per hour, with 8 actual hiking hours spread over a 12 hour day.

Mileage ends up being influenced by two things: hiking speed (which is, in turn influenced by elevation change and pack weight, not to mention physical condition) and the number of actual hiking hours you can crank out. I've got a buddy who can do 25 mile days fairly easily (of course, he's 25, Navy SEAL turned firefighter, and an AT thru-hike veteran) - he maintains a 2 mph clip with a 25 pound pack for 12 hours with no problem. I take the short-cut and meet him at camp. <img src="/forums/images/graemlins/tongue.gif" alt="" />

I don't know what the elevation and terrain are like in the section of the AT you're hiking, but as a starting point, I'd agree with everyone who said use 1.5 miles per hour (don't count breaks as hiking time), and adjust from there for the other factors.

(I can't resist: did I get a better "snarky" score this time? <img src="/forums/images/graemlins/wink.gif" alt="" /> )