I don't believe I'm particularly near any salt marsh, as I live in the suburbs around the Washington, D.C. area. I don't actually see many horseflies in my area, only finding two last year (both already posted). I will leave it to you to decide where this one belongs in the guide. As always, your expertise is appreciated. If you think additional pictures will help, for example a closeup of the head, I may be able to post more images.

Checked all the documentation I have on the lineola complex and have concluded that this specimen is Tabanus subsimilis even though it lacks an obvious reddish tip on the posterior scutellum; its presence/absence is usually the best first character to separate subsimilis from lineola.