This web site has a handy calculator for figuring out miles per day required given start date and end date and zero days.
You can change any of the inputs. Looks like I have to average 15 mpd with up to 15 zero days to complete the hike the way I am planning.
tumbleweed
http://www.geocitiescom/tomjanofsky/at/ATDateApplet.html
13 days to departure to Springer Mt.
tumbleweed