Walk your bikes through the Paw-Paw tunnel. The surface is simply not safe to ride on, even if you have plenty of light.
Take a camelbak (or equivalent). Even one of the small ones ought to be enough of a supplement. The water is not tasty, but nobody in our group 2 years ago got ill from it - mixing Gatorade with the water in your bottles is a really good idea.
If it rains even once in the week before you go, there will be long strectches where you'll be riding through mud.
The gift shop at Sharpsburg has some humongous freeze ice tubes. Two of those make riding up the hill more than worthwhile (the ride back down is pretty sweet too).
There's a stretch of trail on the C & O that's just past a power plant on the other side of the river (can't remember where exactly though - but definitely in the 2nd half of the C & O section). For some reason it is paved with stones that are way larger than what's used anywhere else on the trail. This covers about 2 miles - which seem to last forever. They're a serious threat to cause flats.
Expect to hit lots of roots and downed branches on some sections of the trail. More branches right after any storm blows through. Definitely take spare tubes. Odds are you'll have 2-3 flats along the way.
Enjoy the ride.
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