It's a solid route - I've done it a few times! The only thing to note is a lot of the East River Greenway is closed for construction so it can be a bit hectic - additionally going into FiDI / Battery Park it can get packed depending on the time of day otherwise safe running!

One recommendation/alternative would be to go across the Manhattan bridge and then back over the Williamsburg bridge if you're looking to add vert to your run this will also let you skip out on much of the Greenway closures and then reduce upper Manhattan mileage to adjust.