There are some excellent opportunities for a long beach run in the St. Augustine area. The Atlantic-facing St. Augustine Beach is accessed on Anastasia Island, and runs some 13+ miles north-south. It’s a wide, flat beach with a reliable area of compact sand for running near the water. If you run for 10 miles south, there are views of the Fort Matanzas National Monument. The Anastasia State Park section, at the northern end, has 4 miles of beach running with fairly flat sand.
Vilano Beach, located just 4 miles from St. Augustine over the Vilano Bridge, is another long, sandy beach facing the Atlantic. One can run north for miles and miles, though the beach width varies. The sand is firmest near the water — not quite as reliably compact as St. Augustine Beach, but still a good one.