That task is right up there with making trees for me... blech!
I usually just do a couple of feet at a time, randomly, whenever I can stand it. That's how I got
my previous PRR layout ballasting done (well, done as far as it got before I tore it down... at least the mainline
and main yard were done).
I don't know how anybody can stand to make a whole-hog push to ballast an entire layout as a
steady task, especially one as large as yours. My hat is off to you, man!