chris markley from harrisburg, pa on 5/7/2013 7:54:37 PM:
I rode solo Pittsburgh to Georgetown last October on a traditional steel-frame touring bike (circa 1976), with 700c x 32mm in front and 700c x 35 in back. Back tire was a Schwalbe Marathon Plus -- tough tire. GAP was great. C&O was rougher going. With leaves down, I had to be very careful about bouncing off of tree roots hidden by the leaves. I wouldn't do it with anything smaller than 32mm, and most road bikes these days won't fit even a 32. See what you can fit in your frame. But if clearance is really tight in back and you bust a spoke, the out-of-true wheel will lock up against the chainstays. So either bring spare spokes and a mini cassette remover, or a kevlar spare spoke, or go with thinner tires that give you safe clearance. It's all dependent on what you are comfortable with in your riding style. I've heard some people have done it with 25mm road tires.