I've ridden it.
Road tires will be asking for trouble on the C&O. Any mountain bike tire or tire greater than 1.5 will work. Your mountain bike would be best. It's not only dirt & mud but a lot of rocks you will be riding over. I also recommend fenders on your bike, you will get muddy and the fenders will help keep you a little bit cleaner. Bring a headlight or flashlight for the Paw Paw tunnel. And some on the citizens bike patrol will mention that you are required to have a working bell on your bicycle.
You'll need a rear rack at a minimum to carry your clothes and other items. If you are camping, count on carrying much more gear. There can be long stretches without good water to drink so carry a few bottles of water with you. I don't trust drinking the water from the taps at the hiker-biker campsites. Camping is free along the C&O at hiker-biker campsites. If you are bothered by loud trains, you'll hear plenty of them on your ride.
The distance you will cover is not the same as traveling on a paved road. Riding 60 miles on the C&O is like riding 100 on a paved road. There are going to be stretches where you won't see a person for miles, then there will be sections where you will dodging walkers and joggers every 100 feet.
It's a good ride overall. Plenty of side trips you can take while on the canal like Harpers Ferry, Fort Frederick, Antietam, Shepardstown, Little Orleans. Some good scenery and history but a lot of your ride will look the same.
This is a great site for the C&O canal.
http://www.bikewashington.org
It gives a section by section report on the trail.
Happy trails