My wife and I did it earlier in June on a tandem. Even with 2.125 knobby tires, parts of the C&O were quite an adventure. The advice of going through mud puddles was correct unless you can really tell that the ground is solid if you swerve to avoid the puddle.
The GAP's surface was much like the Katy Trail which I have done 9 times end-to-end. No problem with it.
We did it with a group staying at hotels and having our bags dragged for us. I'm not up to bike packing or hiking like some others. People who can do that really impress me.