A nearly uninterrupted 330-mile bike trail from DC to Pittsburgh gaptrail.org/
Tues 10pm: Someone mentions the C&O/GAP bike combo
Wed 7am: Bought ticket to DC ($74) for Friday morning and ticket from Pittsburgh to Greece ($200) for Monday evening.
Fri 2PM: Bought bike for $85 and biked to C&O Mile Marker 0
Sun 10PM: Arrived in Pittsburgh
Mon 9AM: Sold bike for $100 and got on airport bus 5 minutes later.
Day 1: DC to Harper's Ferry (61 miles)
Tons of history and wildlife. Decent track most of the way, many muddy spots, one catastrophic bridge outage with hike-a-bike rock hop. Two hostels in Harper's ($25 and $35) with free breakfast and cheap backpacker food.
Day 2: Harper's Ferry to Cumberland (125 miles)
Trail quality variable, 5 minutes of rain turned it into a brutal mud slog. Not a single room left in Cumberland or surrounding towns, had to stay at homeless shelter - very friendly people and tasty donated food.
Day 3: Cumberland to Pittsburgh (150 miles)
Excellent fast crushed limestone the whole way. Very scenic with lots of fun bridges and tunnels, and misty mountain tops.
Free campgrounds are available every 5 miles on C&O, but bugs are bad, so you need a tent for this option. Hotels are few and far between and require advanced planning. Some pay campgrounds on GAP.
I think I burned about 20,000 calories on this trip. GAP has lots of ice cream available in trailside shops and frequent restaurants. C&O has fewer options, but you shouldn't need to pack any in.
Every 5 miles on C&O, more random on GAP. One liter capacity should do it.
You don't really need to bring anything -- some bike fixing tools, rain gear. I would advise against bringing a laptop as it is very bumpy and muddy.
A hybrid with a rack is ideal. I had clipless pedals but no shoes, so I did the whole thing in Tevas - but this is the ideal use case for clipless.