That doesn't sound like any fun to me, but to each his own.....
When dry, the GAP is only a little slower than pavement. When wet it can suck. Literally.
The C&O is slower going, with more mud and such.
I wouldn't recommend super skinny tires. Fenders are a good idea. 32mm tires and fenders might be ideal if you can swing that.
Riding so long on such flat terrain is as much of a test of your core strength as anything else.