i also want to ride to DC this summer.
as for march i would say NO WAY, due to winter, and due to spring rain that make it an all out mudfest. I went part of the way in October once and it was totally great then! Lots of people between Connelsville and Ohiopyle though.
My brother gave me this book a couple years ago and I really liked it, though I havent looked at it in a while. There is another one that was just published in October 2009 though. But then I like having a route and a natural history lesson side by side, plus if you go alone you can read about stuff that you are sitting next to.
I would consider going in May or June if I can get the time. Not having tried the full trip, I am not sure how long it most people budget for it.