Go to ATAtrail.org and you can get maps of the GAP. Bike Washington has a detailed C & O site complete with maps.

I think you mentioned you were considering 60 mile days. Starting in McKeesport that breaks down to:

Night 1: Confluence (Outflow Campground)

Night 2: Cumberland (either ride to the first hiker biker site outside town or camp at the YMCA.)

Night 3: Hancock area (one of the hiker/biker sites.)

Night 4. Harpers Ferry area (one of the hiker/biker sites or the city-run campground in Brunswick.)