Rivnuts from Homestead, PA on 3/6/2021 9:51:29 AM:
Option 1: Start in Little Orleans and ride the paved Western Maryland Rail Trail (WMRT) that parallels and is within site of the C&O Towpath itself. The paved WMRT is maybe 23 or 24 miles long. You can stop in Hancock which is about half way along the route. You could then ride back along the natural Towpath itself back to Little Orleans.
Option 2: Start in Shepherdstown and ride to Brunswick and back. Nice trail surface with places to visit and park on either end. That trip is a little less than 20 miles but if you go across the river into Shepherdstown itself, then you'd be at 20 or a little more.
Option 3: Start at White's Ferry and ride to Great Falls. This is a tad over 20 miles each way with places to park on both ends and with interesting sites at each, assuming the ferry resumes operation.
The trail conditions from Shepherdstown to Edwards Ferry are quite good even in rainy weather. The balance of the Towpath is what it is especially in wet weather.
Others will no doubt have other suggestions or favorites for a ride of that length. Lastly, I would avoid the Paw Paw Tunnel area of the trail unless you want to use your gravel bikes for mountain biking of the detour over the closed tunnel path.