One thing to keep in mind when you start riding it is that starting in Pittsburgh it is fairly flat for a while, then trail starts a steady rise. The rise is so shallow that you don't really notice it, but it is steep enough that it does cause you to lose a bit of speed, perhaps one or two mph while you are riding. Thus, on days two or three out of your six day plan, you might start to worry that you will not be able to make destinations within your schedule because your speed might be lower than you anticipated. But once you hit the divide, the trail has a shallow downhill with a higher speed run until you get near Cumberland. Then flatter after Cumberland, but on the C&O you will find that each lock and dam is a sudden short downhill.
My point is that if you are running a bit slow on days two or three going eastbound, that should be expected.
I attached a screen print of the profile that my GPS and GPS software measured from when I rode GAP and C&O, profile should be from Amtrak station in Pittsburgh and end at the HI Hostel in DC with a few deviations where we left the trail for where we stayed at night.
I rode it in May 2013, very few bugs. But I would expect it to be buggy in late Jun or early Jul. Bring whatever you think you need for bugs.
And, when I did it I had accidently turned on my bike light at some inopportune time. When I needed the light to walk through a long dark tunnel, my batteries were mostly depleted. Make sure you have light.
