If your issue is chafing, there are various remedies. A good pair of lycra shorts can help. Anti-chafing cream like Chamois Cream, DZ Nuts, etc., can help, although I've thankfully never had the need to try it (yet).
Personally, I love the Terry Liberator touring saddle (men's version). It has a middle cutout, is firm enough for long days in the saddle, and is well made. In the DC area, you might see if REI or Performance Bike sells it. That way you can try it out and return it easily if it's no good.
One challenge with railtrails like the C&O is that you are likely to be in the same gear and position for long periods, and unless you consciously change up your sitting and hand positions you are asking for trouble, as you don't have the natural variations in terrain that make you subconsciously change position, stand up, etc.
On both my C&O ride and my GAP ride (two weekends ago) my butt was the main thing that hurt at the end of the trip. Not chafing-wise, just being tired. I took to occasionally shifting into a harder gear and doing sprint-and-rests by standing up and pedaling.