My only recommendation would be to give yourself as much time as you possibly can. Pontificating about what will/won't be a feasible pace is pretty much pointless. Injuries, personal emergencies, sickness- there are tons of things which can slow you down, and trying to keep up with such a rigorous schedule will only make things more stressful. Unless you have a real hard deadline on Sept. 1st, set aside another 3-4 week at least.
Starting out with 19 mile days with no experience is definitely not a good idea. If you can comfortably do 12, do 12, then when you can comfortably do more- do more. I know its hard to have faith in the process of getting your trail legs, but it will come with time. Watch "The Whimsical Woman" on YouTube. She started out doing sub 10 mile days, and by Washington, she was crushing 30's.
Good Luck Anon