The C&O runs 180 miles along the Potomac river from Cumberland, MD to Washington, DC. The surface is mostly packed dirt. You don't want to ride this one with skinny road tires. I managed with 700x32 standard touring tires, but riding here is more fun with knobby tires.
I've been on the trail twice, once as part of a Pittsburgh to DC tour, the other on a shorter three-day ride. (We rode the W&OD west for a day out of DC, then turned towards the more-or-less parallel C&O and doubling back towards the district.)
If you're starting in Cumberland, check out the Cumberland Trail Connection, a bike shop that has catered to trail users for years.
The C&O itself starts more or less in the middle of town as a brick-and-stone trail:
Once you exit town, it quickly turns into the dirt trail it will be for most of its way to the nation's capitol.
You'll ride through fields, past pastures and horse farms, and through woods. If it's rained recently, you'll also get pretty filthy. I highly recommend fenders on this trail.
When I went on the northern half of the C&O, portions of it were very overgrown. However, they've since tamed the path a bit, which I think is a shame.
Bring lights along; there are a few unlit tunnels where you'll need them.
Most water available along the trail is from cisterns, and is treated with iodine. If you'll be on the trail for more than a few hours, check the status of these with the park service. Getting stranded overnight without water is, as I can testify, no fun.
If you have time, stop and see the Great Falls. It's right off the trail itself, and maybe an hour or two from the end of the trail in DC. I found it breathtaking, and well worth spending a couple of hours.