Although some believe the Canada side of the Falls is more beautiful, the U.S. side is also spectacular — and wonderful for running. Like the Canada side, a bike path runs alongside the Niagara Scenic Parkway. The prettiest section for running is from the Aquarium of Niagara, heading south to Niagara State Park, with a loop around Goat Island. Here, you’ll get a great view of the falls from Cave of the Winds. This is an out and back run of ~4 miles, including the loop out to and around Goat Island.
Longer Run: A multi-use path runs alongside the Niagara Scenic Parkway for another 4 miles south of Niagara State Park, to I-190 (North Grand Island Bridge). The Parkway is busy, but the path is decently separated and alongside the water. A full out and back is 7.2 miles (11.6 km). Also, you can combine this with the Canada side path — use the Rainbow Bridge, which pedestrians are allowed to cross (about 1 km).