Hey: Many use mnt bikes with knobbies but a road bike with 700x32s will work. Less tread is faster than a lot of knobbies. You don't want to max out the pressure in the tires. Less than maximum will give you a much better ride as the trail can have a few bumps/ruts. A road bike will be much faster than either a hybrid or mnt bike. I prefer to ride it on a touring bike with 700x35s, slick/semi slick tires. As far as comfort goes I think everyone has their preferences. I am much more comfortable on my touring bike than mnt bike for long rides. 12-15mph is considered normal speeds for this trail with many riders much lower. I wouldn't change my bike if it fits comfortably. I would change the tires to 1.50" insted of 1.75". If you have a road bike that can handle a thicker 700(say 32 or 35) then try that route.
it's a nice trails, stop in Harpers Ferry for a custard ice cream(yum), don't worry about the speed. It's friggin hot there now, should hit 100F(38C) by Tuesday of this week.