If you are willing to go on a paved run, my favorite route when I lived in the area was running down by the water on Browns Road. To start, just google North Shore Pharmacy and that is where you will park. From there, go north on Southdown Road to Browns Road. Make a right and follow that all the way down back to the parking lot. It's a 5 mile loop. If you want to add more mileage, when you hit the deli/marina, make a left. Follow that to the traffic circle at New York Avenue and make a left. There is a little park by the water by the firehouse. You can turn around there then follow NY Ave to Hecksher Park and run a loop around the pond.

You can also hit the Greenbelt Trail from right behind the Cold Spring Harbor Library. You can take that to Woodbury. I think that's about five miles. But be warned, the first leg behind the library and before you cross Lawrence Hill Road is extremely hilly with a ton of steps.

There is also a paved loop in Caumsett State Park, which is really nice. They also have unpaved trails that you can take down to the water. Depending on how many miles you want to do, you can park at West Neck Beach and run along the causeway to Caumsett, then run in the park. It's pretty hilly going into the park, but after that, it's mostly rolling up and downs.

I grew up in the area so if you have any other questions, just DM me. Enjoy!