The road surface on the GAP is pretty smooth. You could easily do it on 28mm road tires. 38mm Marathons will be way more than adequate.
if you continue on to the C&O, the surface is rougher, but even there the 38 marathons would be adequate.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.