Chipping in with trail running info. I used to live in the Poolesville/Boyds, MD area, worked in horses, was a runner off and on. Within a 15 minute drive of where I lived, I had access to the C&O Canal, Little Bennet Park, the Seneca Greenway, the Seneca Ridge trail, Hoyles Mill trail, Ten Mile Creek, Black Hills Park, and whatever they call the chaos out the back of the Germantown Soccerplex. Many of those link to each other. For example, you could do Black Hills -> Hoyles Mill -> Soccerplex -> Seneca Ridge -> Seneca Greenway -> C&O… Is 200 miles enough for him? MCRRC (the local running club) is very active, and the running store in the Kentlands is great.
I now live in Prince George’s County, and there are some good trail options (Rosaryville, Jug Bay, Governor’s Bridge, Merkle) but definitely fewer choices. The local running scene seems to be more focused in DC proper or in Annapolis. PG would be the better commute to Navy Yard but for once a week both locations are completely viable.