I don't think the GAP and C&O option is bad. You'd want to abandon the C&O around Williamsport, from where you go over some hills to Gettysburg, then York, Lancaster. Lancaster county is lovely cycling. I wouldn't fear 400 miles in a week, but it depends on your style.
Albany NY is about 250 miles, which would bring you down through (or near) the Catskills and the Delaware River. Might be very nice too.
I won't have much time to train for this one after a month of no biking. 400 miles would be aggressive for me in a week and I know PA isn't flat.
As for the Catskills and the Delaware River, I just did a ride through them last year.