MollyG, the bulk of the trails are in the Greenbelt, which is a collection of parks that all have their own names but probably nobody would know them by those names (Bloodroot Valley? Deere Park? Both are in 'the greenbelt'). The conservancy that cares for the trails has an interactive map: https://sigreenbelt.org/greenbelt-trails-interactive-map/ However, I also have put together a number of routes from the ferry to/through a lot of parkland on SI, including big pieces of the Greenbelt. Here's one from the ferry that is ~15 miles, only ~4 of which are strictly on the street (not even alongside a park). https://www.mapmyrun.com/routes/view/694314438 You'll see at mile 10 or so in the run mapped is the middle of the greenbelt so you can mix and match trails as you like them. Also, you can leave the run I linked to a bit earlier to meet up with the one train line on SI, so that a 10 miler or so from the ferry can bring you to the train that brings you right back to the ferry. I also really, really encourage you to ask REI SoHo for more events on SI. They do stuff in the other boroughs, upstate and in Long Island, but (to my knowledge) not yet anything on SI!
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