I just did the ride from PGH to DC last month and had the time of my life in spite of rain almost constant every day. More to the point... my ride was my 2010 Tricross Triple fitted with fenders and racks/panniers. I worried about tires and gave consideration to using 35mm ContinentalCyclocross Speed tires that are on my older S-Works Tricross. But I opted for a straight touring tire... 32mm Schwalbe Marathon Plus. They were... and are... one great tire and worked wonderfully well for what has been described by trail maintenance folks (C&O) as "the worst we've ever seen it." The Gatorskins will work fine... but the Schwalbe's give you a little more tread, a lot more flat prevention, and a good bit more breadth if the mud persists.

I should note that I was traveling only partially loaded... 7 pounds in the bar bag and 25 pounds in the rear panniers for a total load of 32 pounds give or take. The ride was never harsh. Yes there are roots and mud pits and a loose rock here and there on the C&O but it was far from harsh.

Not terribly difficult. Amtrak comes into 30th Street Station, which is on the western side of Center City. You ride 1-2 miles on city streets to the Ben Franklin Bridge, which crosses over to Camden (has walkways on the side). Camden is kinda dodgy, depending where you ride, but shortly you are out of it and can choose your route to Ocean City. It's actually very pretty riding through South Jersey and the Pine Barrens, etc. Probably about 70 miles all told. The local MS150 ride goes from Cherry Hill, NJ (just outside Philadelphia) to Ocean City yearly, so you can probably find some cue sheets online from their rides (like these: City to Shore Route Options & Rest Stops - National MS Society).

If you don't want to (or don't have time to) ride, NJ Transit has regular buses (think Greyhound type buses) from Philly to OC. The bus depot is in Center City, adjacent to the SEPTA Market East (aka Jefferson) Station. I believe with your AMTRAK ticket you can take a SEPTA Regional Rail train for free two stops from 30th Street to the Jefferson Station. As long as it's off-peak you should be able to bring your bike on board the train with you (although it might be challenging). NJ Transit also offers regular trains from 30th Street Station in Philadelphia (AMTRAK station) to Atlantic City, and then you could easily ride to OC (10 miles or so along the coast).

Only ride where I got saddle-sore from bumps was C&O ride way back in high-school on a thinner-tire bike. C&O doesn't have massive bumps but they're pretty constant. One might like the challenge of riding on a thin-tire bike but otherwise why not go comfy w/50 mm tires?

I have a couple of apps on my phone which I use to find grocery stores. I've found them very reliable from Ohio through Michigan across Ontario and from Niagara Falls to Philadelphia and across Maryland. I've also used the same app in the southern Appalachias with good results. The app showed nothing along the C&O for very long distances without making major detours...I consider a 14 mile round trip to be a major detour on top of a 70 mile day.

There are also long distances on the C&O where you simply can't get off the trail for any reason. As you say, advanced planning helps...like carrying food because you can't just run to the grocery store on a whim.

Perhaps we are talking two different problems. I'm not talking about restaurants and prepared food. I taking about obtaining groceries. Frankly, I seldom eat meals during the hours that I ride. I suffer from too much discomfort when eating high fat meals while I'm actively riding. And, at the end of the day, I'm in camp preparing my food. I usually carry 3 days worth of food wherever I go on tour because I can't depend on finding food just anywhere.

As I said above, the GAP would be easier to find grocery stores because it has more towns along it. The C&O is about as close as I've come to Colorado and Western "wilderness" riding in the eastern US as I've found. It just doesn't have the "civilization" along it that the GAP does.

Because 50mm tires don't fit a lot of frames that aren't mountain bikes. A mountain bike isn't a bad choice for the C&O but it is a bad choice for touring on paved roads.

I typically avoid eating out and try to keep my intake of processed food to a minimum so I can definitely understand where you're coming from.

We have disagreed on several topics but on this I am 100 percent in agreement.

OP Vexxer has a nice stable of bikes, I'd guess a used MTB would be in financial range & most bikers know somebody with little-used MTB to borrow. Anyway if using a road/cross bike I'd think about stuff like extra padding in handlebar tape, saddle etc, maybe even a Thudbuster seatpost Or perhaps forgo C&O & just do GAP plus side loops or whatnot?