Are you close enough that you could do a dry run for a few miles down the path?

How heavy is she... I know, that is always a good question But, it is quite possible that 28mm tires for a 100 pound rider would be like 35 mm tires for a 150 pound rider, and you could drop pressures accordingly.

I haven't been on the Chesapeake & Ohio Canal Tow Path, but I used to ride on the Katy Trail in Missouri. That path had a well packed fine pea gravel (limestone?), and was just fine with 23 and 25mm tires (except just after a snow melt).

Anyway, what are your road conditions like?

Selection of good "gravel" tires for 700x28 are a bit limited, but there should be a few.