Misc notes:
I'd go 6 days instead of 5.
Harper's Ferry is an attractive stop, distance-wise. Unfortunately, to actually get to HF from the C&O trail, you have to carry your bike up a very unpleasant outdoor spiral staircase. I'd consider Point of Rocks (where I admittedly haven't stayed myself) or Shepherdstown instead. Shepherdstown is pretty bike-friendly.
If you've never ridden the C&O trail, don't expect it to be like the GAP. IMO, it's much less scenic. It's also less fun to ride on. The GAP is mostly crushed limestone and asphalt. The C&O is mostly mud, rocks, and tree roots. Don't expect to go as fast on the C&O, especially if it's raining.
Standard caveats: be sure to take tools and spares. You will be far from help at many points. I recommend a couple of spare tubes and spokes, pump, standard multi-tool, and a chain tool. Large portions of the GAP and the C&O have no mobile phone coverage.
You may want to check out the Western Maryland bike route as an alternative to the C&O trail. I will probably do that next time I go.
Think about food and water.
Others have complained about insects on the C&O trail. I never had a problem, and didn't need any repellant. YMMV.
A good rule-of-thumb for the "in shape" check is that you should do a ride that's roughly the distance you plan to cover in one day. You should feel fine the next morning. If you feel drained the next morning, then you probably can't sustain that mileage for a week.
Don't take the Rankin bridge. It's under construction right now, and it's terrible for any vehicle, but especially for a bicycle.
You don't need to go to McKeesport to pick up the GAP - you can pick it up in Duquesne - it's right at the RIDC entrance on Grant. If you turn off 837 into RIDC, it's immediately on your right. Bonus reason for picking up the GAP there: you get to ride over the beautiful Riverton bridge. Getting to Duquesne from the city is currently unpleasant - the trail to connect up to Homestead isn't open yet, AFAIK. I did this ride earlier this summer, and I just started in Duquesne. I've ridden 837 before, and it sucked. I had no desire to start my fun trail ride with a sucky highway ride.
Be sure that your bike has a headlight and a blinky. Even if you don't ride at night, you're riding in the dark in the tunnels. You'll want a headlight that's capable of providing genuine illumination as well as visibility to others. If you choose to go through the Paw Paw tunnel (rather than take the detour over and around), you'll want either a flashlight or a detachable headlight on your bike.
If you're looking at lodging in Connellsville, the "Melody Motor Lodge" advertises itself as trail-accessible, and it's the only place in Connellsville that advertises in the GAP Trail Book. The ride from the trail to the motel is extremely unpleasant. Once there, you're on a highway that I wouldn't want to ride on in the dark, so factor that into schedule and dinner plans. I will not go back there. Next time, I'll either find someplace else or just camp.
Others have reported problems
Check out;
The Great Allegheny Passage... anyone?
Things to know about the C&O towpath
DC Bike Trip
Books to check out:
An Uncommon Passage
The C&O Canal Companion